How to Keep Target Spot of Cucurbits in Your Sights
Target spot of cucurbits, or Corynespora blight, is caused by the fungus Corynespora cassiicola and is worldwide in distribution. It is a relatively new disease in Florida, first reported on cucumber from the Immokalee area in 1967.
The name target spot derives from the ringed, or bull’s eye, appearance that is sometimes seen in lesions caused by C. cassiicola. However, concentric rings are not always readily apparent in target spot lesions, and not all lesions with concentric ringing are caused by C. cassiicola. It is often necessary to examine suspected target spot lesions for the characteristic spores of the causal fungus to ensure a correct diagnosis.
On cucumbers, the disease starts as small, yellow leaf flecks that gradually enlarge to about 0.4 inches across and become angular. Individual mature lesions are light tan with a thin brown margin. Lesions may coalesce, with the development of large circular areas of dead tissue that dry and tear out, giving a shredded appearance to the leaf. Left unchecked, the disease can defoliate and destroy an entire crop. Small, elongated lesions may occur on cucumber petioles and stems.
In the early stages, target spot is difficult to distinguish from angular leaf spot and downy mildew, two common foliar diseases of cucumber. In late stages, the disease can be confused with anthracnose of cucumber.
The number of host plants attacked by C. cassiicola totals more than 60 species, with new hosts reported regularly. Among vegetable crops, C. cassiicola infects beans and other legumes, peppers, cucumbers and other cucurbits, and tomatoes.
Survival and Spread
Corynespora has several means for survival and spread in the field. It can survive up to two years in crop debris and on weed hosts. The wide host range of this fungus also contributes to survival of the fungus in Florida. Despite its wide host range, strains of Corynespora from different hosts are not always cross-infective to other host species.
The primary means of field spread is by air-disseminated conidia that have a primary release period in mid-morning. The spores are spread by wind-blown rain, and if windy, wet weather continues for a few days, spread can be rapid. Warm temperatures are favorable for disease development, although infection readily occurs under cooler temperatures.
Cultural controls such as crop rotation, control of weedy hosts, good sanitation (including rapid and timely destruction of crop debris), and use of clean transplants are important. To avoid spreading the disease, growers should refrain from working in the crop when leaves are wet.
Target spot is primarily controlled by applying protectant fungicides. It is important to provide good coverage. Because of frequent changes in pesticide registrations, consult UF/IFAS recommendations for currently labeled fungicides for target spot control in Florida cucurbits.
Resistant cultivars of cucumber are available and should be used in conjunction with the above-mentioned disease management strategies.