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Annual Reports

1995UC IPM Competitive Grants Program

Decision Support

The UC IPM Project will consider projects whose aim is the development and promotion of decision systems useful in crop production and pest management. General areas of such work might include but are not limited to development or improvement of sampling or detection methods, quantification of crop loss, development or improvement of damage thresholds or action levels, risk assessment, economic or other evaluation of IPM programs, development or evaluation of expert systems or other computer models.

Development of monitoring programs for pests and biological control agents involving sampling decision rules, control action thresholds, or improved methods for quantifying pest abundance or potential crop loss can significantly reduce the riskiness of pest control, reduce the number of pesticide applications, and improve adoption of new practices. Control action threshold research should be based on the relationship of pest occurrence or abundance to measurable crop loss in order to ensure cost-effective decisions.

Modeling or expert systems proposals must specify the intended audience for the model, clearly identify the model's utility to that audience, and present a well-formulated plan for making it available to potential users. A proposed model structure must be included. High priority will be given to proposals that clearly define objectives; will produce programs that can be easily and directly used in the implementation of integrated pest management; will focus on a biological system that has had sufficient preliminary investigation to allow completion and validation of the program in a reasonable amount of time; and will involve principal investigators who have demonstrated experience in systems modeling or systems management. Authors are required to prepare a user's manual and technical documentation for all programs, and the proposal must discuss how this will be accomplished.


New Projects Funded for 1995-96
Continuing Projects Funded for 1995-96
Projects that Ended in 1994-95
Final Reports for Projects that Ended in 1994

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New Projects Funded for 1995-96

Evaluating Strategies for Managing an Insect-Transmitted Plant Virus

The incidence of tomato infectious chlorosis virus in greenhouse and field tomatoes and management strategies for its control. (Year 1 of 1; $10,000)

Principal Investigators: B. W. Falk, Plant Pathology, Davis; S. R. Temple, Agronomy and Range Science, Davis; M. Davis, Plant Pathology, Davis

Objectives:
Develop and utilize environmentally sound crop management strategies for plant disease and pest control, related to the whitefly-transmitted tomato infectious chlorosis virus (TICV).
Assess the incidence and importance of TICV in California tomatoes.
Identify factors and conditions affecting the incidence and spread of TICV in California tomatoes.
Assess the effectiveness of tomato seedling transplant management practices to control TICV in California tomatoes.


Molecular Markers for Monitoring Lettuce Downy Mildew

Development and use of molecular markers for the control of lettuce downy mildew. (Year 1 of 1; $20,000)

Principal Investigators: R. W. Michelmore, Vegetable Crops, Davis; K. V. Subbarao, Plant Pathology, Davis; S. T. Koike, UC Cooperative Extension, Monterey County; F. F. Laemmlen, UC Cooperative Extension, Santa Barbara County

Objectives:
Use reliable DNA fingerprints from PCR-based, molecular markers for lettuce downy mildew to monitor downy mildew populations in California.
Characterize the patterns of spread through lettuce growing regions.
Determine the amount of variation within commercial fields.
Provide information for the optimal use of resistant cultivars and fungicide treatments.


Citrus Thrips Decision-Making

Risk assessment for fruit scarring by citrus thrips Scirtothrips citri (Moulton) from soil and air temperatures before petalfall. (Year 1 of 2; $19,086)

Principal Investigators: H. Schweizer, Entomology, Riverside; J. G. Morse, Entomology, Riverside

Objectives:
Develop a temperature-driven, phenological model for estimating the amount of fruit economically scarred by citrus thrips, which can be used to decide whether or not to apply control measures (e.g., insecticides) against citrus thrips, Scirtothrips citri (Moulton).
Develop the model specifically for navel oranges in the San Joaquin Valley and for selective (IPM compatible) pesticides, but with applicability for other varieties and regions.

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Continuing Projects Funded for 1995-96

Improving Nematode Sampling Strategies

Sequential decision methods to control sugarbeet cyst nematode via rotations. (Year 3 of 3; $27,728)

Principal Investigators: B. B. Westerdahl, Nematology, Davis; O. R. Burt, Agricultural Economics, Davis; E. P. Caswell-Chen, Nematology, Davis

Objectives:
Establish a damage threshold for sugarbeet cyst nematode on sugarbeets in northern California.
Refine and extend the theory and methods of critical point (damage threshold) models for controlling nematodes by means of cropping sequences.

Summary of Progress Approximately 228,000 acres of sugarbeets distributed among 29 counties are grown each year in California. The cash farm income reported in 1987 for sugarbeets was 213 million dollars. In 1978, 366,565 acres in California were reported to be infested with the sugar beet cyst nematode (SBCN) (Heterodera schachtii). A three to four year rotation to nonhost crops is the currently recommended management program for SBCN on sugarbeets. This crop rotation program is widely practiced in the Imperial Valley where profitable rotation crops are available. On the other hand, growers in infested areas of northern California are in need of improved management programs, because of the lower profitability of all available rotation crops (typically beans, wheat and corn) except tomatoes, combined with the experience of growers that longer rotations are needed. Northern California sugarbeet growers cannot reliably practice crop rotation without: 1) a knowledge of the rate of population decline under nonhost crops, 2) a damage threshold for the two most commonly practiced planting regimes (late spring planting with overwintering of beets followed by spring harvest, or early spring planting followed by fall harvest) and, 3) improvements in the economic criteria for choosing an optimal sequence of crops.

In northern California, we have been repeatedly sampling 10 fields for the past 6 years and are seeing population decreases of 20 to 25 percent per year. Development of damage thresholds and improvements in economic criteria for planning crop rotations are the goals of this project. Field and microplot experiments are in progress to develop a damage threshold for different planting dates. We have been comparing different field sampling strategies and methods to detect SBCN in soil samples to try to improve on the sensitivity of previously used techniques. To date, larval extraction and bioassays, although taking less time, have not proved more sensitive than traditional cyst and egg extraction methods. An economic model that can be computed on a hand calculator has been developed to determine the SBCN population density at which sugarbeets should be planted again after a sequence of nonhost crops. This model will be tested and refined during the succeeding two years.


Tracking Leafminers in Lettuce

Study of Liriomyza leafminer outbreaks in lettuce. (Year 2 of 2; $16,422)

Principal Investigators: J. Granett, Entomology, Davis; W. E. Chaney, UC Cooperative Extension, Monterey County

Objectives:
Identify sources of Liriomyza infestations in California lettuce production areas.
Distinguish relative contributions of specific mortality factors to the overall mortality of Liriomyza in the field.

Summary of Progress Leafminers in the genus Liriomyza have been increasing as problems in lettuce crops in many areas of California. Studies were initiated to identify possible factors responsible for the rise of these leafminers to major pest status. An understanding of the proximate causes of this increase in leafminer populations is a prerequisite to the development of intelligent IPM strategies. Three possible sources of leafminer populations exist: migration into the field from adjacent weeds and/or crops, over-cropping as puparia in the soil, and within-field development. Yellow water pan traps located around the perimeter of each of the fields were used to detect migration into the crop. Yellow sticky cards positioned uniformly within each field and examination of whole lettuce plants served to evaluate within-field population development. Emergence cages positioned in each field determined the number of adults emerging within a crop, and also allowed an evaluation of the crop to crop survival of leafminers.

The collection of data was recently completed from four 16 acre lettuce fields. Although these data have not been subjected to statistical analysis, some general observations can be made. Despite repeated applications of insecticides, some of the which are purported to be very effective against leafminers, very high leafminer populations were detected in all fields. These applications undoubtedly contributed to the fact that no natural enemies were found parasitizing larvae from any of the fields. Each lettuce field used in our study was preceded by a lettuce crop; however, the number of over-cropping leafminers was greater when there was a shorter time between harvesting and planting the new crop. For example, with a turnaround of only 15 days, as many as 23,000 leafminers (extrapolated from emergence cage data) were emerging from the soil per day in a newly planted lettuce field. With a turnaround time of >30 days, few leafminers were found emerging from the soil.

The number of flies collected in water pan traps appears to be related to the harvest of nearby crops (i.e., lettuce, celery, etc.) which serve as a reservoir for large leafminer populations. In all fields the within-field population buildup occurred gradually and reached high levels at the end of the crop.

Data collection will occur from an additional two fields this spring. These data will be particularly important because these fields will represent the first lettuce plantings in the Salinas Valley and the initiation of leafminers in the lettuce fields.


Downy Mildew Warning System

Implementation of a disease warning system for downy mildew of lettuce using high resolution weather forecasts and a geographical information system. (Year 2 of 2; $25,000)

Principal Investigator: A. H. C. van Bruggen, Plant Pathology, Davis

Objectives:
Couple an existing infection model for lettuce downy mildew with high-resolution weather forecasts and with an existing simulation model for leaf wetness duration to generate field-specific predictions of daily infection risk, and validate these predictions against field data at selected locations in the coastal valleys.
Develop a Geographical Information System (GIS), incorporating data layers for topography, inoculum availability, and weather and leaf wetness forecasts, to calculate daily infection risk for any location in the coastal valleys.
Develop methods to automate the input of weather forecasts and up-to-date information about downy mildew inoculum availability into the GIS, and implement the GIS-based disease warning system using UCIPM and UCCE resources.

Summary of Progress The overall objective of this research is to develop a warning system for improved chemical control of lettuce downy mildew based on weather forecasts. The use of forecast weather, rather than measured (past) weather, could provide sufficient lead time for growers to schedule and perform spray applications before infection takes place, thus improving the efficacy of protective fungicides. Our previous studies in commercial lettuce fields in coastal California had shown that infection with downy mildew occurs mainly during periods of prolonged morning leaf wetness duration (LWD), suggesting that fungicide applications could be timed according to morning-LWD. Therefore, this research project is focusing on predicting periods with prolonged morning-LWD using weather forecasts, and on the field validation of leaf wetness forecasts and downy mildew control by fungicide sprays scheduled according to predicted morning-LWD. A simulation model for LWD caused by dew was used in conjunction with numerical weather forecasts from the National Meteorological Center to predict prolonged morning-LWD periods (and thus disease risk) with a lead time of 24 hours. In seven validation trials, more than 90% of the days were correctly classified as days with or without prolonged morning-LWD. However, the exact onset and ending of the leaf wetness periods was not predicted accurately enough. This lack of accuracy was probably due to the fact that meso-scale national weather forecasts were used for leaf wetness prediction and that dew was the only form of leaf wetness considered in the simulation model. All wet periods that were missed in the forecasts were due to fog drizzle which was not included in the simulation model. Recently, we compared leaf wetness predictions from our simulation model using high-resolution weather forecasts (from MESO, Inc., Troy, New York) with specific, local leaf wetness forecasts provided by a local weather forecasting company (Fox Weather, Oxnard, California) and measured leaf wetness at three locations. Forecasts of onset and ending of leaf wetness periods have improved considerably (in particular by Fox Weather), but still need additional improvement before these forecasts can be used for disease warning systems.


Novel Approaches to Aphid Control

Integrated management of aphids and aphid-vectored virus diseases in vegetable crops. (Year 2 of 3; $25,129)

Principal Investigators: J. J. Stapleton, Statewide IPM Project, San Joaquin Valley; C. G. Summers, Entomology, Davis/Kearney Agricultural Center, Parlier

Objectives:
Determine and compare the effectiveness of combinations of reflectorized plant coatings, soil mulches, and the aphid alarm pheromone in delaying and/or reducing the development of aphid-borne virus diseases and subsequent crop loss in susceptible vegetable varieties.
Determine the extent to which these various methods and combinations repel aphids from plants thus delaying and/or reducing colonization and virus transmission.
Determine the effects of these treatments on earliness, health, yield, and economics of susceptible vegetable crops.

Summary of Progress A complex of aphid-vectored virus diseases has caused major losses in many vegetable crops grown in the inland valleys of California during recent years. Three field experiements were conducted in 1994 to test the effectiveness of reflectorized, spray, and polyethylene soil mulches, plant coatings, and the aphid alarm pheromone, alone and in combination, for management of aphids and aphid-borne virus diseases in San Joaquin Valley vegetable crops. One experiment each was done using cantaloupe melons, zucchini squash, and bell pepper as test crops. Excellent responses of melon and zucchini were obtained to the reflectorized mulches under severe virus conditions. Aphid numbers on mulched plants were consistently lower than on those which were nonmulched. Onset of symptoms of cucumber mosaic, watermelon mosaic, and zucchini yellows mosaic viruses were delayed 3-6 weeks, which is critical to obtain normal flowering and fruiting. The high virus disease incidence reduced cumulative marketable squash yields (fruit number and fresh weight) in the control plots to near zero. Both spray and polyethylene mulch treatments provided the difference between total crop loss and a successful crop, increasing the cumulative number of fruit produced per plot from 2 to 44-45, and the cumulative fresh weight per plot from 1 lb to 9-10 lbs. Likewise in cantaloupe melon, all mulched treatments gave large increases in yield over the nonmulched controls, ranging from 4.4-fold (25% bed width mulched ) to 25.2-fold (polyethylene 100% bed width mulched). Results with bell pepper were inconclusive, and the experiment will be repeated in 1995. No significant benefit from plant coatings or the aphid alarm pheromone was obtained on any of the crops.


Gray Mold Protection for Kiwi

Using colonization levels of kiwifruit sepals and stem ends to predict gray mold and reduce fungicides. (Year 2 of 2; $11,870)

Principal Investigator: T. J. Michailides, Plant Pathology, Davis/Kearney Agricultural Center, Parlier

Objectives:
Determine if incidence of colonization of kiwifruit sepals and stem ends by Botrytis cinerea in vineyards can be a predictor of Botrytis gray mold decay of fruit in storage.
Use the predictor (findings in objective 1 above) to minimize the use of preharvest vinclozolin (Ronilan) sprays for controlling Botrytis gray mold.

Summary of Progress Gray mold storage decay caused by Botrytis cinerea is the most important disease of kiwifruit, even though B. cinerea is not considered a field pathogen in California kiwifruit because the disease does not occur in the field. Postharvest decay by gray mold, however, is commonly responsible for large losses during long-term storage of kiwifruit, and storage decay is a direct result of B. cinerea infections that do occur in the field but remain latent in the senescent floral parts (mainly sepals) and stem end scars (receptacles). In this project we are trying to determine whether the incidence of colonization of sepals and stem ends by B. cinerea can be a predictor of the amounts of gray mold in storage. We cannot comment officially on the results of 1994, since recording of postharvest decay in storage will be done after 3 months (in January) and 5 months (in March); however, we can safely say that at least the preliminary results from 1992 and 1993 were successful in showing that the incidence of colonization is a good predictor of the gray mold in storage. From the isolation results, we know that the vineyards fell into the three categories of the level of gray mold as expected, based on their histories of gray mold in storage.

Bloom and preharvest sprays of vinclozolin (Ronilan 50W) have been shown to decrease significantly the incidence of latent infections by B. cinerea and thus to reduce the incidence of postharvest gray mold. (Ronilan is registered for control of Botrytis rot on kiwifruit.) Although best control has been achieved with two bloom and two preharvest sprays, almost equal control was achieved with single bloom and single preharvest applications. To help reduce fungicide applications, we plan to determine whether two vinclozolin preharvest applications are redundant in vineyards with low or medium disease potential.

All experiments were performed as planned, and five boxes of fruit from each treated and nontreated vine were stored awaiting evaluation in mid January (3 months storage) and again in mid March (5 month storage). Preliminary experiments in 1992 and 1993 suggest that there is a direct relationship between sepal or fruit stem colonization by Botrytis cinerea and incidence of gray mold in storage, findings on which this proposal was based.


Silverleaf Whitefly in San Joaquin Valley

Phenology, seasonal host selection, and biology of the silverleaf whitefly (Bemisia argentifolii Bellows & Perring) in the San Joaquin Valley. (Year 2 of 3; $34,853)

Principal Investigators: C. G. Summers, Entomology, Davis/Kearney Agricultural Center, Parlier; R. L. Coviello, UC Cooperative Extension, Fresno County; M. J. Jimenez, UC Cooperative Extension, Tulare County

Objectives:
Determine the seasonal phenology and population dynamics, the seasonal host plant acceptability, and the crop host selection sequence of the silverleaf whitefly on selected hosts in the San Joaquin Valley.
Determine the over wintering biology of silverleaf whitefly and the role of selected crop and non-crop hosts in overwintering survival.
Establish threshold values for nymphal populations causing broccoli white stalk and irregular ripening of tomatoes in the San Joaquin Valley.

Summary of Progress The silverleaf whitefly has continued to increase its distribution and range throughout the San Joaquin Valley. It currently infests all agricultural areas of Kern, Kings, Tulare and Fresno counties. During 1994, infestations were found in Madera and Merced counties, but they are confined to relatively small areas at this time.

In mid-March 1994, the earliest naturally occurring silverleaf whitefly populations were found on cucumbers in the Kearney Agricultural Center host sequence study. Individuals (probably "red-eye pupa" which over wintered on broccoli in these plots) gave rise to the initial adult population infesting the cucumbers. Squash and tomatoes were found infested ca. one month later. All were planted in February. The earliest populations found in agricultural areas of the Valley were on February planted tomatoes in Five Points (late May). Spring planted vegetables appear to be a key in the development of early whitefly populations.

Population build-up on all crops occurs slowly during the spring and early summer, probably in response to mild temperatures. Melons, cotton and later planted tomatoes were infested in June and July. Populations build rapidly in response to increasing temperatures in July and August. Maximum populations on all crops are reached late in the summer, usually September or October. Those crops which mature late in the summer are at the highest risk for injury. The host sequence study conducted at Kearney shows that all susceptible crops will be colonized as soon as they emerge. There is an obvious difference in host acceptance in warm season crops with melons > pima cotton > acala cotton > tomatoes > black-eye beans > alfalfa. Broccoli and cauliflower are the most acceptable winter crops. Beets are not an acceptable host in the San Joaquin Valley.

Sowthistle, prickly lettuce, spurge, mustard and cheeseweed are thus far good overwintering hosts. Broccoli, cauliflower and alfalfa appear to be good overwintering crop hosts. Viability of eggs, nymphs, and "red-eye pupa" was near 100% on all hosts in early December.


Rice/Weed Simulation Model

Management-oriented simulation of rice growth and weed competition. (Year 2 of 3; $26,392)

Principal Investigators: T. C. Foin, Environmental Studies, Davis; J. E. Hill, Agronomy and Range Science, Davis

Objectives:
Continue experimental research on rice-weed interactions that 1) advances the search for the characteristic properties that are relatively independent of the identity of the weed species; and 2) provides the parameter values necessary for the simulation models.
Complete the revisions to the California Rice (CARICE) simulation model needed to adapt it for predicting weed impacts and to evaluate alternative strategies for weed control.

Summary of Progress UCD greenhouse competition experiments with Ammannia spp. (redstem) and Cyperus difformis (smallflower umbrella plant) were repeated in 1994. Results paralleled those from earlier trials in 1993 and 1992. Both weeds significantly reduced rice tiller and panicle density, above-ground drymass, and grain yield at densities as low as 61 weeds/m.

An experiment was run in which rice planting date was constant, and redstem seeds were sown at 0, 8, 13, 18, and 23 days after planting (DAP). At harvest (75 DAP) weed plants had exceeded mean rice height only in 0 and 8 DAP treatments. Weed density, drymass, and seed production decreased significantly with delayed germination. Because redstem that germinated at 13 DAP or later were unable to penetrate the canopy, herbicide control of redstem should be targeted for two weeks to one month after rice planting. A uniform rice stand should suppress later-germinating redstem.

A field experiment was conducted in Butte County on redstem control. Herbicide treatments included bensulfuron (Londax, 1.25 oz. ai) at 11 DAP (control), 32 DAP, and 57 DAP. The grower also applied MCPA at 55 DAP. At harvest (130 DAP) there was a mean weed density of about 32 weeds/m2 in both control and 32 DAP treatments. The application method used for bensulfuron at 32 DAP (aqueous solution applied directly into the water) proved ineffective.

Weed density declined 50% after a Londax treatment as a foliar spray at 57 DAP, and rice grain yield increased by 8%. MCPA provided complete weed control, but reduced rice yields almost 11% below controls due to rice injury. Pre-plant rolling practices (causing widely-spaced "rows" of rice) may have delayed canopy closure, contributing to redstem establishment and rice yield loss.

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Projects that Ended in 1994-95

Monitoring Citrus Cutworm

Improved monitoring methods for citrus cutworm.

Principal Investigator: E. E. Grafton-Cardwell, Entomology, Riverside

Summary of Accomplishments: During the spring of 1994, pheromone traps that attract male citrus cutworm moths were placed in 15 orchards divided between two geographical regions (Exeter and Ivanhoe areas) of San Joaquin Valley citrus. We observed that male moth capture in the traps occurred from January 11 through May 7 in 1994, similar to the pattern seen in 1993. The major peak of male moth activity in all sites was during the period of March 1 through March 25 in the Exeter area and extended into April 18 in the Ivanhoe area.

Two groups of 25 eggs each were placed in growth chambers and held at 10, 15, 20 and 25°C. Eggs hatched in all temperature situations; however, larvae in the chamber that was set for 10°C did not mature past the 2nd larval instar. Larvae in the other three temperatures matured into pupae and the rate of development increased with increasing temperature. Using regression analysis we estimated that the baseline temperature for citrus cutworm egg hatch and larval development is 48.2°F (9.0°C). We can use this baseline temperature to begin to accumulate degree day units (DD) in the field to predict larval emergence.

In seven of the 1994 orchards, we monitored the accumulated degree day units above the 9.0°C threshold for egg and larval development starting from the peak in the moth flight. We found that larvae began to appear in significant numbers after 200-250 DD had accumulated in the commercial orchards. We also found that the majority of the larvae entered pupal cells in the soil by 550-600 DD. The differences in dates between peak male flights between sites were very small and the accumulated DD were very similar between sites. This suggests, that with a few temperature recorders located throughout the citrus growing region of the San Joaquin Valley we can predict the initiation of egg hatch and larval emergence for most orchards.

Where the second male moth flight in the Ivanhoe area was extended into mid-April, we observed greater emergence of the larvae in late April. This late larval emergence is thought by pest control advisers to be the most damaging because it can occur after petal fall when the small fruit can be scarred. Better prediction of larval emergence will improve timing of microbial insecticides and improve their effectiveness because small larvae are most easily killed.


Identifying Nematodes with RAPD

Use of random amplified DNA polymorphisms for root-knot nematode biotype identification.

Principal Investigators: V. M. Williamson, Nematology, Davis; E. P. Caswell-Chen, Nematology, Davis; B. B. Westerdahl, Nematology, Davis

Summary of Progress Root-knot nematodes (RKNs) are the major nematode pests in California field and vegetable crops. There are several RKN species and biotypes that differ in the plant hosts that they parasitize. We are developing a simple DNA-based assay to distinguish species and biotypes that differ in host range. The ability to rapidly distinguish RKN species and biotypes is essential for optimal management decisions.

We initiated our studies with eight different RKN cultures, including the five species that are major problems in California and three atypical populations that are able to infect nematode-resistant tomato plants. Isozyme electrophoresis, morphological analysis and host range tests were used to confirm the species identity of each population. DNA isolated from eggs of each population was subjected to a technique called RAPD (pronounced "rapid") analysis to identify differences unique to each population. We found many differences between the five major RKN species. However, DNAs from the biotypes of the same species have been very difficult to distinguish. This suggests that these nematodes are genetically very similar to each other.

We have been using a new DNA-based assay to identify the species present in field samples sent to us by farm advisors. This new assay can be used to identify the species of individual juveniles and, thus, does not require that adult female RKN be present. This has allowed us to identify the species in diagnostic samples where only juveniles are present. Previously we were able to identify the species precisely only when adults were present. Identification with the new assay is completely consistent with the results from our current isozyme electrophoresis technique. Because some of the steps in the procedure are tedious, we have been developing a simpler technique using information that we obtained from the RAPD assays. We have started by developing a positive diagnostic test for M. hapla, the northern RKN. We have confirmed the usefulness of this test by obtaining isolates from seven locations in North America. Validation of the assay for the other species is currently in progress. On several occasions we have analyzed samples of heavily galled roots from commercial fields planted to nematode resistant tomatoes and found that these fields contained either M. hapla (which infect resistant tomato), or a mixture of this and other species. The M. hapla diagnostic test that we are developing should provide a convenient method for screening fields to be planted to resistant tomatoes.


Rice Water Weevil Affects Yield But Not Maturity

Influence of rice seeding method and rice variety on damage potential and economic threshold of rice water weevil.

Principal Investigators: A. T. Palrang, Entomology, Davis; L. D. Godfrey, Entomology, Davis

Objectives:
Evaluate the relationship among rice water weevil adult density, adult feeding scars on rice plant leaves, larval density and injury on roots, and rice plant growth/yield, and derive a modified economic threshold from these studies.
Evaluate the effects of rice seeding technique (water-seeding and drill-seeding) and rice variety (rice water weevil tolerant and susceptible) on rice water weevil damage potential to rice.
Determine the influence of rice water weevil larval injury on rice plant physiological response and relate this response to grain yield.

Summary of Accomplishments: Rice plant response to rice water weevil (RWW) damage was evaluated in two rice establishment systems (water-seeded and drill-seeded) and in two rice varieties ('M-202' and PI 506230). Controlled infestations of RWW larvae were developed by introducing various levels of RWW adults (0, 0.2, 0.4, and 0.6 adults per rice plant) into aluminum enclosure ring plots. The influence of RWW injury on rice growth/development, maturity, photosynthetic rate, and grain yield was studied.

Larval densities in the water-seeded 'M-202' averaged 1.2 and 8.2 per plant in the 0 and 0.2 to 0.6 RWW per plant treatments, respectively. High larval mortality occurred, because of poor early-season root growth and subsequent larval starvation, in the high infestation treatments. This resulted in no significant differences among the three infestation regimes, which was in contrast to the 1993 study.

Rice growth and development was significantly affected by RWW damage. As in 1993, plant tillering was significantly affected; uninfested 'M-202' plants peaked at ~7 tillers per plant compared with only 4.5 tillers per plant in the high infestation regimes (0.4 and 0.6 RWW per plant). Similarly, plant height was reduced by 22% by RWW damage.

Rice maturity was not significantly effected by RWW larval feeding. This is consistent with the 1993 data and is in contrast to the common belief regarding RWW damage. In the water-seeded 'M-202' plots, grain yield was highest in the uninfested, intermediate in the 0.2 RWW per plant infestation, and lowest in the 0.4 and 0.6 RWW per plant infestations. On a percentage basis, grain yields in 'M-202' were reduced by ~15 and 25% in the 0.2 and 0.4/0.6 RWW per plant treatments, respectively. The yield reductions were less than those recorded in 1993. In 1993, the 0.6 RWW per plant infestation reduced the grain yield by 45% in 'M-202' and by 28% in PI 506230.

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Final Reports for Projects that Ended in 1994

Leaf Wetness and Mildew

Forecasting of downy mildew on lettuce.

Principal Investigator: A. H. C. van Bruggen, Plant Pathology, Davis

Summary of Accomplishments: Downy mildew of lettuce is usually controlled with calendar-based sprays of protective fungicides (maneb or fosetyl-Al). Despite regular fungicide use, downy mildew sometimes causes substantial losses in commercial lettuce production. At other times, disease pressure is low and most or all fungicide applications are unnecessary. In a two-year field study in the coastal valleys of California, we found that infection of lettuce by Bremia lactucae occurred primarily on days when leaf wetness ended later than 10:00 am. Weather variables other than leaf wetness were not consistently associated with infection. Environmental conditions were generally conducive for sporulation of the pathogen; thus, a warning system based on sporulation would not be useful. Since the infection stage was limited by morning leaf wetness, our downy mildew warning system will be based on predictions of leaf wetness in the morning. We conducted 7 field trials to test whether fungicide sprays (maneb and fosetyl-Al) timed according to predicted morning leaf wetness would give improved downy mildew control and/or lead to a reduction in the number of applications compared to a calendar-based spray schedule with three sprays per crop. Leaf wetness forecasts were generated using a dew simulation model with numerical weather forecasts from the National Meteorological Center as input. More than 90% of all days were correctly classified as days with or without a prolonged morning wet period. All wet periods that were missed in the forecasts were due to fog drizzle which was not included in the simulation model. The total number of sprays in the seven trials was reduced by more than 50% relative to the calendar-based schedule. There was no significant difference between the two fungicides and spray schedules in disease incidence or severity (except for one trial in Santa Maria, see below). Final disease levels were too low to generate differences in yield. In a study comparing different irrigation practices, we showed that the daily leaf wetness periods were longer and disease severity higher in fields under furrow than under drip irrigation, indicating that the microclimate may influence disease development when the mesoclimate is marginally conducive for infection. Thus, the network of weather stations and/or forecast locations needs to be fairly high for an accurate disease warning system. Research is currently underway to improve our forecasting system by using high-resolution weather forecasts as input for the dew model and considering leaf wetness caused by drizzle. In addition, the risk of infection will also be related to the availability of inoculum.

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