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Project description

Ecologically-based management of vine decline of melons caused by Monosporascus cannonballus. (00FE003)
Program UC IPM competitive research grants program
Principal
investigator 		M.E. Stanghellini, Plant Pathology, UC Riverside
Host/habitat Melons
Pest Monosporascus cannonballus
Discipline Plant Pathology
Review
panel 		Applied Field Ecology
 Start year (duration)  2000 (Three Years)
Objectives Develop a post-harvest crop destruction method(s) that will inhibit reproduction of Monosporascus cannonballus on infected melon roots.

Final report Ascospores of Monosporascus cannonballus function as primary inoculum for infection of melon roots. Our previous studies demonstrated that pathogen reproduction (i.e., ascospore production) occurs on infected melon roots primarily after the crop has been harvested. Thus, the key to maintaining low soil populations densities of the pathogen is to destroy the hyphae of the pathogen in infected roots as soon as possible after crop termination, thus inhibiting ascospore production. Relative to the nontreated controls, our field results demonstrated (in 2001, 2002, and 2003), that an immediate postharvest application of metam sodium (applied via the drip irrigation system at 20 gal/A) or cultivation (which lifts roots onto the surface of the soil) significantly inhibited pathogen reproduction (as evidenced by the number of roots bearing perithecia) in infected melon roots. Inhibition of pathogen reproduction was also reflected in the significantly lower soil populations of the pathogen in the metam sodium and cultivation treatments compared to the control treatment. Our results also demonstrated that two different postharvest residue destruction methods used by growers to kill the plants at crop termination, (foliar application of an herbicide or mechanical destruction of melon vines) were counterproductive in that they stimulated pathogen reproduction, rather than the intended inhibitory response. Additionally, results from our trials in commercial melon fields conclusively demonstrated that a single preplant soil fumigation (chloropicrin) to reduce the amount of primary inoculum in soil, followed by a postharvest application of metam sodium at the end of each three consecutive growing seasons (spring 2001, fall 2001, and spring 2002), provided significant disease control (i.e., reduction in the percentage of roots with lesions and perithecia) not only for the first crop, but for the next two consecutive melon crops planted on the same beds.

Third-year
progress 		Ascospores of Monosporascus cannonballus, the causal agent of vine decline of melon, function as primary inoculum for infection of melon roots. Our studies indicate that the development of an effective disease management strategy is dependent upon (i) reducing the amount of primary inoculum in soil and then (ii) maintaining low pathogen populations in soil by inhibiting pathogen reproduction. Our studies indicate that pathogen reproduction occurs on melon roots primarily after the crop has been harvested. Thus, destruction of the melon roots as soon as possible after harvest will inhibit pathogen reproduction. We identified (in 2000), and verified (in 2001 and 2002), that an immediate postharvest application of metam sodium or cultivation (which lifts roots onto the surface of the soil) inhibited pathogen reproduction and that these postharvest strategies resulted in a significant reduction is root disease severity in the 2002 spring crop. Objectives for 2003 include documentation and verification that inhibition of pathogen reproduction on roots translates into efficacious disease control. Additionally, results from our trials in commercial melon fields conclusively demonstrated that a single preplant soil fumigation (chloropicrin) to reduce the amount of primary inoculum in soil, followed by a postharvest application of metam sodium at the end of each three consecutive growing seasons (Spring of 2001, Fall 2001, and the Spring of 2002), provided significant disease control (i.e., reduction in the percentage of roots with lesions ) not only for the first crop but for the next two consecutive melon crops planted on the same beds.
Second-year
progress 		Preplant fumigation of soil with either methyl bromide or chloropicrin, when compared to nonfumigated soil, resulted in significant reductions in the incidence and severity of melon root infection caused by M. cannonballus.

Post-harvest treatments applied at the end of the Spring 2001 crop at DREC verified our hypothesis that little or no pathogen reproduction occurred on roots of plants that were either (i) cultivated immediately after harvest or (ii) that were treated with metam sodium immediately after harvest. In contrast, significant increases in pathogen reproduction (perithecia and ascospore formation) were recorded within 1-2 weeks on roots of control plants that were left in the field after harvest. Evaluation of the efficacy of these postharvest treatments on reductions in disease incidence and severity, coupled with documentation of their effect(s) on the rate of recolonization of soil by the pathogen, will be evaluated at the end of the 2002 Spring crop (i.e., year two of a three-year study).

Additionally, results from our trials in commercial fields indicate that preplant soil fumigation followed by an immediate postharvest application of metam sodium provided significant disease control not only for the first crop but also for a second consecutive crop planted on the same beds (Fall crop, 2001). A third consecutive melon crop on the same beds is planned for the Spring of 2002.

First-year
progress 		Our field results, obtained from spring 2000 melon crop in replicated plots located at DREC (Holtville, Calif.), showed that significant pathogen reproduction occurs within one to two weeks on melon roots left in the field after the final harvest. We also identified two post-harvest crop destruction methods that exhibit promise in inhibiting reproduction of the pathogen on infected melon roots left in the field after harvest. Specifically, pathogen reproduction of melon roots left in the soil after harvest of the crop was (a) totally inhibited by an application of metam-sodium via the drip irrigation system and (b) significantly reduced (90-98%) by cultivation and air-drying of the infected roots. On the other hand, destruction of the foilage either by physical (trashing of the vines) or chemical methods (herbicide application to the leaves) increased the rate of pathogen reproduction on the roots of melon plants. Many growers currently use the latter two crop destruction methods. Both of these methods, as well as no action by the grower, actually increase the population of the pathogen in soil.

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