Charcoal Rot

Charcoal rot affects all cucurbits. First symptoms are yellowing and death of crown leaves and water-soaked lesions on the stem at the soil line. As the disease progresses, the stem of infected plants ooze amber-colored gum, and the stem eventually becomes dry and tan-to-brown in color. The stem may be girdled by the lesion, resulting in plant death. Numerous microsclerotia, visible as black specks, are embedded in the dead plant tissue.

Macrophomina phaseolina is a soilborne fungus occurring in most soils in California. The fungus persists in soil as microsclerotia for 3 to 12 years and can infect 500 plant species. The pathogen most commonly infects melon stems at the soil line within 1 to 2 weeks after planting, but the first disease symptoms occur late in the growing season, usually within 1 to 2 weeks of harvest.

The fungus is a stress pathogen and disease incidence increases with increases in water stress, a heavy fruit load, and high temperatures. Although severe charcoal rot is relatively uncommon in furrow- irrigated fields; ironically, its prevalence has increased with the use of buried drip irrigation systems. This may have occurred as a result of increased salt levels (stress) in beds, particularly at the soil surface. Additionally, disease incidence and severity is most common in fields cropped multiple times to melons.

Start looking for charcoal rot during the vegetative growth stage, and note infections to make management decisions for the next crop. Rotation to a nonhost crop for 2 to 3 years can be an effective disease management strategy in some crop production systems. However, avoidance of drought stress throughout the growing season is paramount to disease management. Leaching soil to reduce salinity levels, particularly at soil surface layers, may help reduce the incidence of disease in drip-irrigated fields. Further, destruction of infected plant tissue before the pathogen reproduces at the end of the growing season will prevent a buildup of soil inoculum. The use of grafted transplants (i.e., susceptible scions grafted onto resistant cucurbit rootstock) has been proposed as an effective management strategy for the control of charcoal rot as well as many other soilborne root-infecting pathogens where the use of chemicals is not feasible. No preplant or postplant chemical control measures have been reported. Solarization is not promising for diseases favored by heat like charcoal rot.