Vine Decline (Crown Blight)

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.

Monosporascus cannonballus is indigenous to the southwestern U.S. 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 per gram of soil. Commercial melon fields with a known history of this disease contain as few as 2 ascospores per gram of soil.

For 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.

Start looking for symptoms of vine decline during the vegetative growth stage. Solarization is not a promising technique for diseases favored by heat like vine decline. Postplant soil treatment with fludioxonil (Cannonball), a fungicide, may suppress disease development. Additional management strategies may include, when appropriate or cost effective, the use of grafted melons on resistant squash rootstock. Rotation out of melons will significantly reduce but not eliminate soil inoculum. Some cantaloupe varieties are tolerant to this pathogen, and some varieties, such as Caravelle and Desert Mark, are more susceptible than others.