Enlarged target

In the harvest (2013/2014), some cotton crops in western Bahia, northern Mato Grosso do Sul and the regions of Campo Verde, Primavera do Leste and Brasnorte in the state of Mato Grosso developed a foliar disease with symptoms similar to target spot of soybeans, caused by the fungus Corynespora cassiicola (Berk. & Curtis) Wei. The target spot, under favorable environmental conditions, can cause defoliation and, consequently, economic losses in soybeans. In cotton, the disease was reported in the USA in 2012 in the state of Georgia, mainly in the cultivar DP 1048 B2RF with defoliation estimated at 70% (2).

The initial symptoms of target spot on cotton are small, circular, purplish spots on the leaves. As the disease progresses, these spots become rounded or irregular shaped spots, with dark brown edges and a light brown center, varying in size from 2mm to 20mm. The lesions, when fully developed, may present concentric rings and, when the infection is high, the leaves fall prematurely. Always associated with symptoms, a fungus with simple conidiophores was isolated, which originated sub-hyaline conidia, with a variable number of septa (four to 20) with dimensions from 50μm in length to 190μm in length and 7μm in width to 16μm in width, slightly curved and obclavate to cylindrical in shape. Based on the similarity of symptoms and signs already described in the literature (2), the tentative diagnosis points to Corynespora cassiicola (Berk. & Curtis) Wei., same causal agent of target spot in soybeans.

This fungus causes disease in 530 plant species from 380 genera, including monocots and dicots (1). In addition to phytopathogenic isolates, C. cassiicola includes saprophytic isolates. The fungus has also been reported to cause skin infections in humans (3).

The ideal environmental conditions for the development of C. cassiicola are known in some cultures. In tomato plants, the ideal temperature is between 20ºC and 28ºC and more than 16 hours of leaf wetness. In rubber trees, the ideal conditions are temperatures between 25ºC and 30ºC with humidity above 90%. For cucumber and tobacco, the optimum temperature for the development of the fungus is between 27,5ºC and 30ºC. In general, mild temperatures (below 30ºC) and prolonged periods of high humidity favor the development of the pathogen.

Wind is the main route of dissemination of the pathogen over short distances and there is still no information about its spread through cotton seeds. The fungus, being cosmopolitan and necrotrophic, can survive on host plants and colonize cultural remains of various plant species (4).

Given the occurrence of these symptoms in different cotton producing regions and at epidemic levels, it is necessary to devise strategies to anticipate the problem. Initially, Koch's postulates are being conducted to prove C. cassiicola as a causal agent of target spot in cotton.

Given the severity observed in soybean cultivation and now in cotton in the cerrado cultivation areas, some questions arise and must be answered by research, such as: I) There were variations in the population of C. cassiicola in Brazil? II) Could the increase in inoculum in soybean crops be affecting subsequent cotton cultivation (second harvest) or in the next agricultural year (single harvest)? III) Is the use of fungicides based solely on triazoles and strobilurins on cotton aimed at controlling ramularia spot sufficient to control the target spot? IV) Cotton cultivars recently introduced because they have features Are transgenics more susceptible than germplasm adapted to the tropical environment? V) How and for how long does the pathogen survive in a tropical environment? VI) Are there isolates of the pathogen in Brazil that are naturally resistant to the various fungicides used on soybeans, cotton and corn, as has already been demonstrated for benzimidazoles, strobilurins and carboxamides in other countries? VII) Is there genetic resistance in cotton germplasm? VIII) Which species can be used in rotation to reduce the initial inoculum of C. cassiicola? These questions, once answered, helped to elucidate the epidemiology of the disease and thus assist in defining management.

This article was published in issue 184 of Cultivar Grandes Culturas magazine. Click here to read the edition.