Contribuições ao estudo da interação de Corynespora cassiicola com sementes de algodão e soja

Abstract

Corynespora cassiicola, the pathogen responsible for target spot in cotton and soybean, can be transmitted to multiple hosts through infected seeds. This study aimed to evaluate the relationship between this pathogen and cotton and soybean seeds, with a focus on seed-to-plant transmission and the resulting effects of this interaction. To determine the transmission rate and assess the effects of C. cassiicola on the performance of one cotton cultivar and two soybean cultivars of seeds and seedlings, we used two-isolated fungus and inoculated them using the water conditioning method. For cotton, the seeds were kept on the fungal colonies for 36, 72, 108 and 144 hours, equivalent to the pathogen inoculum levels established for this study, with denominations: NI36, NI72, NI108 and NI144, respectively. For the soybean seeds, we maintained physical contact with the fungal colony for 36, 72, and 108 hours, corresponding to inoculum levels NI36, NI72, and NI108. Additionally, chemical treatment with the product (Metalaxyl-M + Fludioxonil) was applied at two inoculum levels, NI72 and NI108. The experiments were conducted in plant growth chambers at temperatures of 20 °C and 25 °C, with daily evaluations of the effects. The overall transmission rate of each pathogen was determined by summing the pre-emergence mortality rate and the transmission rates for both symptomatic and asymptomatic infections. The total transmission rates of C. cassiicola were directly proportional to the inoculum levels, regardless of the isolate or cultivation temperature. Physiological effects were assessed by measuring germination, vigor (emergence speed index), initial and final stand, plant height, fresh and dry weight, electrical conductivity, and disease/damage index. The health was assessed by blotter test on seeds inoculated at the same inoculum levels used to assess pathogen transmission rates. It was observed that for both crops under study, higher initial inoculum levels in the seeds resulted in more pronounced negative effects on the quality of the inoculated seeds and the germinated plants. Plants from seeds with the NI144 inoculum level for cotton and NI108 for soybean exhibited the lowest values of EVI, stands, weights, and plant heights. The seed quality tests showed that higher inoculum levels were associated with a lower percentage of normal seedlings in the germination test, which also corresponded to an increased incidence of the pathogen in the seeds at these levels. Infected seeds exhibited advanced tissue degradation in their structures, as indicated by electrical conductivity analyses. Comparative evaluations of the performance of soybean seeds revealed that chemically treated seeds were effective in reducing the level of the pathogen when compared to untreated seeds. By employing the conventional PCR technique and incubating plant tissue fragments, it was demonstrated that asymptomatic plants can harbor the pathogen, which may become apparent during the crop cycle in a cultivated environment.