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Research and IPM

Models: Diseases

Crop: Pear

Disease: Scab
Pathogen: Venturia pirina

Note: Before using a model that was not field tested or validated for a specific location, the model should be tested for one or more seasons under local conditions to verify that it will work in the desired location. See "Validation Work" below.

Model 1 of 4

Model developer and citation

Spotts, R. A. and Cervantes, L. A. 1991. Effects of Temperature and Wetness on Infection of Pear by Venturia pirina and the Relationship Between Preharvest Inoculation and Storage Scab. Plant Disease 75:1204-1207.

Sensor location

Within the orchard, at 1.5 meters height in a standard weather shelter.

Input variables

Environmental: Temperature, wetness duration.

Model description

Spotts and Cervantes present data from a controlled environment experiment with pear seedlings as well as in-field limb bagging experiments on the effects of temperature and wetness duration on conidial infections of pear seedlings, leaves and fruit. They have not evaluated ascospore infection conditions but suggest that they should be quite similar to conidial infection conditions and therefore their model can be used to predict primary infection by ascospores.

Spotts and Cervantes' model predicts leaf infection by conidia of V. pirina based on a regression of the hours of wetness duration and temperature during those wet hours:

Y = 66.82 - 8.72X + 0.44X 2 - 0.0076X3 (R2=0.967).

Where

Y = minimum hours of wetness for infection

X = temperature in degrees Celsius

Action threshold

Model developers observed that the minimum wetness duration required for foliar infection by conidia all fell between the values required for "light" and "moderate" infection of apple by V. inequalis ascospores according to the Mills table. Therefore when using the Mills table for pear scab ascospore or conidia infection, the authors recommend the use of hours of wetness for "light" infection to be more conservative.

Current limitations

This model was based on work in Oregon and needs further validation in California. The model is based on work from one season, further validation of the use of the conidial infection model for ascospore infection events would be valuable.

Future directions

The authors are working on developing a Potential Ascospore Dose (PAD) value for pear scab similar to the PAD developed by MacHardy for apple scab. They hope to combine the infection model with their ascospore maturation model and a PAD value in an integrated disease risk model.

Related work

Spotts, R. A. and Cervantes, L. A. 1994. Factors affecting maturation and release of ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264.

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Model 2 of 4

Model developer and citation

Spotts, R. A. and Cervantes, L. A. 1994. Factors affecting maturation and release of ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264.

Sensor location

Within orchard, at 1.5 meters above the ground in a standard weather shelter. Leaf wetness was also monitored at 2 mm above pear leaves on the ground.

Input variables

Environmental: Temperature, precipitation, leaf wetness duration.

Calculated: degree-days with a base temperature of 0 C.

Pathogen: Date of first mature ascospore observation.

Model description

Spotts and Cervantes linear regression model predicts maturation of ascospores of V. pirina based on accumulated degree-days over a 0 C ( 32 F) threshold. Degree day accumulation begins when the first mature ascospore is observed. Model parameters are:

ln(1/1-Y)=-0.00797 + 0.00415X

Where

Y = proportion of mature asci

X = accumulated degree-days, base 0 C

This model only predicts ascospore maturity, however the authors provide three seasons of weather data and related ascospore release events. They demonstrate that releases are correlated with rain and dew events but they do not propose a specific model to predict release based on weather variables.

Action threshold

This model predicts maturation of ascospores but the authors have not combined it with infection models to time applications of fungicides for disease control.

Model validation

Spotts, R. A. and Cervantes, L. A. 1994. Factors Affecting Maturation and Release of Ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264.

The ascospore maturation model has been evaluated by B. Zoller in Mendocino County (pers. comm.)

This ascospore maturation model in conjunction with the Mills table for predicting infection periods is currently being validated in California by D. Gubler and G. McGourty in Mendocino and Lake counties.

Current limitations

The model lacks information on disease development as it is related to ascospore maturation and does not provide disease management recommendations.

The model developers recommend that duration of wetness events which trigger ascospore releases be measured starting at initial wetness regardless of the time of night or day as ascospores have been trapped during day and night time wetness events.

Future directions

This model will be combined with the same author's infection model to aid in on-farm disease management.

Related work

Spotts, R. A. and Cervantes, L. A. Effects of Temperature and Wetness on Infection of Pear by Venturia pirina and the Relationship Between Preharvest Inoculation and Storage Scab. Plant Disease 75:1204-1207.

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Model 3 of 4

Model developer and citation

Doug Gubler and Glen McGourty are currently evaluating a combination of the Spotts and Cervantes model for ascopsore maturation together with infection conditions from the Mills table developed for apple scab. They are also looking at the effect of high temperature and low moisture on pseudothecial development in order to incorporate these factors into a model of ascospore maturation.

Spotts, R. A. and Cervantes, L. A. 1994. Factors affecting maturation and release of ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264.

Spotts, R. A. and Cervantes, L. A. 1991. Effects of Temperature and Wetness on Infection of Pear by Venturia pirina and the Relationship Between Preharvest Inoculation and Storage Scab. Plant Disease 75:1204-1207.

Mills, W. D. 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab. Cornell Ext. Bull. 630.

Sensor location

Within orchard.

Input variables

Environmental: Temperature, precipitation, leaf wetness duration.

Calculated: degree-days with a base temperature of 0 C.

Pathogen: Date of first mature ascospore observation.

Model description

Gubler and McGourty of the University of California have proposed the intregration of the Mills tables and the pear scab ascospore maturation model of Spotts and Cervantes to forecast infection risks of Venturia pirina on pears in California.

The Spotts and Cervantes model predicts maturation of ascospores of V. pirina based on accumulated degree-days over a 0 C ( 32 F) threshold. Degree day accumulation begins when the first mature ascospore is observed. Model parameters are:

ln(1/1-Y)=-0.00797 + 0.00415X

Where

Y = proportion of mature asci

X = accumulated degree-days, base 0 C

The Gubler McGourty model then determines three levels of infection risk by ascospores as a function of daily average temperature and leaf wetness duration, based on the Mills table developed for apple scab ascospore infection. The time required for conidial infection is one-third shorter than that for infection by ascospores.

Action threshold

Under development.

Model validation

Spotts, R. A. and Cervantes, L. A. 1994. Factors Affecting Maturation and Release of Ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264.

The ascospore maturation model has also been evaluated by B. Zoller in Mendocino County (pers. comm.)

Current limitations

The model is still under development. It uses the Mills table developed for apple scab infection to predict infection by the related pear scab pathogen.

Future directions

McGourty and Gubler are studying environmental conditions leading to pseudothecial maturation and specifically the effect of high temperature and low moisture in order to incorporate these factors into a model of ascospore maturation.

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Model 4 of 4

Model developer and citation

Mills, W. D. 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab. Cornell Ext. Bull. 630.

Sensor location

Not given.

Input variables

Environmental: Temperature, leaf wetness duration.

Model description

This model predicts three levels of infection risk by ascospores as a function of daily average temperature and leaf wetness duration, based on the Mills table (see below) developed for apple scab ascospore infection.

Mills table

Hours of wetting required for V. inequalis ascospore infection of apples

Average temperature(F) Light infection Moderate infection Heavy infection
33-41 over 48 __ __
42 30 40 60
43 25 34 51
44 22 30 45
45 20 27 41
46 19 25 38
47 17 23 35
48 15 20 30
49 14.5 20 30
50 14 19 29
51 13 18 27
52 12 18 26
53 12 17 25
54 11.5 16 24
55 11 16 24
56 11 15 22
57 10 14 22
58 10 14 21
59 10 13 21
60 9.5 13 20
61 9 13 20
62 9 12 19
63 9 12 18
64 9 12 18
65 9 12 18
66-75 9 12 18
76 9.5 12 19
77 11 14 21
78 13 17 26
Note: The time required for conidial infection is one-third shorter than that for infection by ascospores.

Action threshold

According to the model, treat when an assessment of the level of infection risk exceeds personal risk.

Model validation

Lewis, F. H. 1943. Studies on spray and dust schedules for control of apple scab in western New York. Ph.D. thesis, Cornell University, Ithaca, NY.

Model implementation

This model is being implemented by pear growers in Mendocino and Lake counties, CA.

Limitations

This model does not determine the presence of initial inoculum and therefore it can over or under-estimate the risk of infection.

Related work

Mills, W. D., and LaPlante, A. A. 1951. Diseases and insects in the Orchard. Cornell Ext. Bull. 711, rev. 1951.

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