How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Vine decline of melons, caused by the root-infecting ascomycete Monosporascus cannonballus, is characterized by the sudden and generally uniform canopy collapse of entire fields 1 to 2 weeks before harvest. The most susceptible hosts are cantaloupe and watermelon and, to a lesser extent, honeydew melons. The disease is particularly severe in the hot and semi-arid production regions of the world. In California, the disease is most prevalent in the Imperial Valley, Coachella Valley and the Palo Verde Valley. However, it occurs sporadically in the mid- and lower San Joaquin Valley. The rapidity and severity of collapse are generally associated with high ambient temperatures, heavy whitefly/leafminer infestations, heavy fruit loads, and water stress.

First symptoms include chlorosis and necrosis of older crown leaves and occasionally wedge-shaped necrotic areas on crown leaves extending to petiole. Within several days of the first foliar symptoms, the canopy collapses. At the time of canopy collapse, root symptoms in most commercial fields are generally lacking. However, within days following plant death, roots lesions will become evident on primary and secondary roots, followed by the production of numerous perithecia (small, black, round structures that measure 0.5 mm in diameter and protrude from the dead tissue) within the next 7 to 14 days.

COMMENTS ON THE DISEASE

Monosporascus cannonballus is indigenous to the southwestern US. The host range of this fungus in the field is restricted to cucurbits. The fungus persists in soil as ascospores that serve as the survival and primary source of inoculum for root infection. Although ascospores can germinate and colonize roots at temperature as low as 68°F, the optimum is between 75° to 85°F. In a spring-planted crop, root infection occurs about 57 days after planting, whereas in a late spring or fall-planted crop, root infection can occur within 9 days after planting.

Disease incidence and severity is most pronounced in crops maturing during late May to mid-September. Pathogen reproduction in infected roots occurs primarily within 7 to 14 days following plant death. The root system of a single infected plant can support the production of 400,000 ascospores. The latter population, if incorporated into a cubic foot of soil via cultivation, would result in a population of 10 ascospores/gram of soil. Commercial melon fields with a known history of this disease contain as few as 2 ascospores per gram of soil.

MANAGEMENT

Although there is no immunity in cantaloupe to this pathogen, some varieties, such as Caravelle and Desert Mark, are more susceptible than others.

In fields with a known history of vine decline, preplant soil fumigation significantly reduces the resident population of the pathogen in soil as well as the percentage of roots infected by the fungus. However, because fumigation does not eliminate the pathogen, the residual population will infect, reproduce on infected roots, and buildup the population following consecutive cropping to melons. Thus, destruction of infected roots immediately after the final harvest is critical to maintenance of low soil populations of the pathogen (not only in fumigated soils but also in fields that have little or no history of the disease). An immediate postharvest application of metam sodium (applied via the drip irrigation system) or cultivation (which lifts the roots onto the soil surface for rapid desiccation) will inhibit pathogen multiplication in infected roots and prevent a build up of inoculum (ascospores) in soil.

Postplant soil treatment with fludioxonil (Cannonball), a fungicide, may suppress disease development. Additional management strategies include, when appropriate or cost effective, the use of grafted melons on resistant squash rootstock.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
Not all registered pesticides are listed. Always read label of product being used.
A.	METAM SODIUM*
	(Vapam, Sectagon)	Label rates	48	NA
	COMMENTS: Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
B.	CHLOROPICRIN*	Label rates	48	NA
C.	FLUDIOXONIL	Label rates	12	14
	(Cannonball)
	MODE OF ACTION GROUP NAME (NUMBER1): Phenylpyrrole (12)
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1	Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
NA	Not applicable.

PUBLICATION

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445
Diseases
R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Imperial County
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Diseases:
W. D. Gubler, Plant Pathology, UC Davis

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