Disease Cycle, Development and Management of Sclerotinia Stem Rot of Potato

Abstract

Potato is severely affected by Sclerotinia sclerotiorum in the US Pacific Northwest (PNW) and satisfactory disease management has not been achieved until recently. Recent research has given a better understanding of key epidemiological factors which has resulted in improved disease management in the PNW. This work reviews the epidemiology of Sclerotinia stem rot and highlights information that has led to the better management of the disease on potato. The primary source of inoculum of Sclerotinia sclerotiorum for potato in the Columbia Basin of the PNW are ascospores produced within potato fields or carried by wind currents from neighboring fields planted to winter cereals or other crops. Ascospores are ejected from apothecia and disseminated throughout the Columbia Basin over an extended time period. Immature apothecia generally first emerge at or shortly after row closure in potato fields in the Columbia Basin. Ascospores of S. sclerotiorum are incapable of direct infection of intact green potato tissues, and flower blossoms are crucial for infection and development of the disease in potato. Airborne ascospores are deposited on open potato blossoms still attached to the canopy. Infested flowers fall and are trapped on stems, usually leaf axils, or fall on the ground, and fungal mycelia then rapidly colonize the blossoms when humidity is high in the plant canopy. Ascospores are also deposited on senescent and dead plant material on the ground, germinate, and produce mycelium. Infection occurs shortly after contaminated blossoms become lodged on stems in the plant canopy, or after stems come in contact with contaminated fallen blossoms or decomposing plant tissues on the ground. Infection can occur within 3 days after contact of contaminated blossoms with green tissue. Lesions initially appear on potato stems 14 to 30 days after row closure and 12 to 20 days following full bloom of primary inflorescences of potato in the Columbia Basin. Significant levels of outcrossing sexual recombination have been found in the homothallic S. sclerotiorum population in the Pacific Northwest. Sclerotinia stem rot of potato is best managed by using an integrated approach of combining cultural practices that produce optimum plant foliar development without producing an excessive crop canopy, irrigation management to avoid excessive amounts of irrigation water, and timely applications of fungicides. Contaminated flower blossoms served as a bridge for infection and fungicides applied before contaminated blossoms drop on foliage most effectively reduce infections on stems. Improved disease management has been achieved by timing fungicides to coincide with full bloom of primary inflorescences.