Target Spot is the name often used for vegetable diseases caused by the fungus Corynespora cassiicola. The name target spot derives from the ringed or bull's eye appearance 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 ring 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 that a correct diagnosis is made. The pathogen can be induced to sporulate from diseased tissue after incubation in a high-humidity chamber for 24-48 hours. In some cases, growth of C. cassiicola from pieces of suspect tissue on agar media in the laboratory is needed to verify that target spot is, indeed, the problem.
In addition to tomato, this fungus has a wide host range and may attack such diverse crops as papaya, passion-vine, pepper, cowpea, cantaloupe, squash, and snap beans, as well as a number of common ornamentals.
The pathogen has several means for survival and spread in the field. It may survive up to two years in crop debris. The wide host range of this fungus also may contribute to survival of the fungus in Florida. The primary means of spread in the field is by airborne conidia.
Optimum conditions for disease development include temperatures from 68°F to 82°F and long periods of free moisture.
In trials, wounding was essential for reproduction of the fruit symptoms.
Target spot is frequently misdiagnosed in its early stages as symptoms are difficult to recognize and can be confused with bacterial spot and early blight. The name derives from the bull’s-eye appearance that is often displayed in lesions caused by the disease. Since concentric rings are not always visible and not all lesions with concentric rings are target spot, it is recommended a laboratory diagnosis be obtained to ensure that a correct diagnosis is made.
On tomato leaves and stems, foliar symptoms of target spot consist of brown-black lesions with subtle concentric rings giving them a target-like appearance. These can sometimes be confused with early blight. On tomato fruit, lesions are more distinct. Small, brown, slightly sunken flecks are seen initially and may resemble abiotic injury such as sandblasting. As fruits mature, the lesions become larger and coalesce, resulting in large pitted areas. Advanced symptoms include large deeply sunken lesions, often with visible dark-gray to black fungal growth in the center. A zone of wrinkled looking tissue may surround the margins of lesions on mature fruit.
Placing suspect fruit in a moist environment for 24 hours will often induce the growth of dark-gray mycelia providing telltale diagnostic evidence of target spot infection.
Strategies for the management of this disease require an integrated approach for best results. Currently, target spot is controlled primarily by applications of protectant fungicides. It should be noted that tank-mix sprays of copper fungicides and maneb do not provide acceptable levels of target spot control. In recent trials at the University of Florida, fungicides were rated for efficacy as follows:
Survival And Spread
In addition to tomato, this fungus has a wide host range and may attack such diverse crops as papaya, passion-vine, pepper, cowpea, cantaloupe, squash, and snap beans, as well as a number of common ornamentals.
The pathogen has several means for survival and spread in the field. It may survive up to two years in crop debris. The wide host range of this fungus also may contribute to survival of the fungus in Florida. The primary means of spread in the field is by airborne conidia.
Optimum conditions for disease development include temperatures from 68°F to 82°F and long periods of free moisture.
In trials, wounding was essential for reproduction of the fruit symptoms.
Symptoms
On tomato leaves and stems, foliar symptoms of target spot consist of brown-black lesions with subtle concentric rings giving them a target-like appearance. These can sometimes be confused with early blight. On tomato fruit, lesions are more distinct. Small, brown, slightly sunken flecks are seen initially and may resemble abiotic injury such as sandblasting. As fruits mature, the lesions become larger and coalesce, resulting in large pitted areas. Advanced symptoms include large deeply sunken lesions, often with visible dark-gray to black fungal growth in the center. A zone of wrinkled looking tissue may surround the margins of lesions on mature fruit.
Placing suspect fruit in a moist environment for 24 hours will often induce the growth of dark-gray mycelia providing telltale diagnostic evidence of target spot infection.
Management
- Switch (cyprodinil and fludioxonil), Inspire Super (difenoconazole and cyprodinil) both from Syngenta Crop Protection
- Revus Top (mandipropamid and difenoconazole, Syngenta Crop Protection) and Scala (pyrimethanil, Bayer CropScience)
- Tanos (famoxadone and cymoxanil, DuPont Crop Protection), Endura (boscalid, BASF), Quadris (azoxystrobin, Syngenta Crop Protection) and other strobilurins, and Reason (fenamidone, Bayer CropScience)
- Bravo (chlorothalonil, Syngenta Crop Protection)
- Mancozeb, Copper.
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