Target spot management in cotton

Typical in soybeans and emerging in cotton crops, the target spot has been observed since the 2011/12 harvest in Mato Grosso do Sul. In 2015, the presence of typical symptoms was verified in an experimental area conducted by Embrapa Agropecuária Oeste, in the municipality of Dourados . Understand how cultivars react to the fungus Corynespora cassiicola It is important to understand the behavior of the disease and establish appropriate prevention and control measures.

The target spot, caused by the fungus Corynespora cassiicola (Berk & Curt.) Wei is a typical soybean disease, reported in this crop for the first time in Brazil in 1974. Currently, it is found in practically all soybean growing regions in Brazil, causing economic losses. C. cassiicola, as a necrotrophic pathogen, survives in crop residues and its main form of dissemination is infected seeds. As a polyphagous fungus, it has been recorded in more than 50 genera of plants, such as soybeans, cowpeas, sesame, tomatoes, cucumbers and more recently in cotton.

In cotton cultivation, target spot was recorded for the first time in Brazil in 1995, in crops in Mato Grosso, and is considered an emerging disease, causing significant losses. In Mato Grosso do Sul, the first record occurred in the 2011/2012 harvest, in cotton crops in the Chapadões region. Symptoms on cotton plants are similar to those seen on soybeans. On cotyledonous leaves, they appear as small circular spots. In the foliar phase of the disease, symptoms first appear on the leaves of the plant, quickly spreading to the middle third and then to the upper third of the plants. Under conditions of high incidence and severity, severe defoliation can also be observed. On the leaves, the symptoms are initially characterized by the presence of brown spots, with a yellowish halo, evolving into large circular spots, light brown to dark brown, forming concentric rings of darker color, with a spot in the center of the lesion. .

In 2015, cotton plants of the cultivars BRS 369 RF, FMT 709 and FM 975 WS, showing typical symptoms of target spot, were observed in the experimental area of ​​Embrapa Agropecuária Oeste, in Dourados, Mato Grosso do Sul. The sowing of the experiment where these plants were found was carried out on 05/11/2014, and the emergence occurred on 12/11/2014. The spacing between rows was 0,90 m, with a sowing density of 10 plants/m. Fertilization was 400 kg/ha of 8-20-20 (N-P-K) at sowing and 250 kg/ha of 20-0-20 (N-P-K) at top dressing. Assessments of incidence (Figure 1) and severity (Figure 2) of the target spot were carried out in these three cotton cultivars. Regarding the incidence of the disease (Figure 1), all evaluated plants of the three cotton cultivars showed symptoms of the target spot, that is, 100% incidence, which is expected for diseases that attack the aerial part of plants and whose pathogens They survive in crop residues and are spread by rain, wind and contaminated seeds. The severity of the target spot (Figure 2) was greater in the lower third compared to the middle third, in the three cultivars evaluated. A difference in behavior was observed between the three cultivars tested in relation to severity (considering the results from the lower and middle thirds). The lowest disease rates were observed for the cultivar FM 975 WS, followed by FMT 709 with intermediate values. The greatest severity of the target spot was observed for the cultivar BRS 369 RF.

The infected leaves were collected and taken to the laboratory for analysis. Isolation of the pathogen from these leaves that showed typical symptoms of the target spot was carried out according to the following procedure: small fragments of the leaves removed from the lesions were initially disinfected with 70% alcohol and 2% sodium hypochlorite. They were later transferred to Petri dishes containing Potato-Dextrose-Agar (BDA) culture medium. Ten days after incubation in a growth chamber (BOD) at 22ºC and a 12-hour light/12-hour dark photoperiod, the fungus developed under these conditions was identified as being Corynespora cassiicola. Identification was carried out based on the morphological characteristics of the fungus: appearance and color of colonies in culture medium and shape, size, number of septa and color of conidia.

To carry out the pathogenicity test, tests were carried out in a greenhouse. Healthy cotton plants, 10 days old, of the DP 555 cultivar, grown in pots, were inoculated by deposition of the inoculum suspension (4 x 10⁴ conidia/ml) with a manual sprayer on the cotton sheets, until the point of drainage. The control was atomized only with water and kept under the same environmental conditions. After inoculation, the plants were kept in a humid chamber for a period of 48 hours, when they were removed from this condition. Eight days after inoculation, symptoms similar to those observed in cotton plants in the field were identified in the inoculated plants. Control plants, sprayed with water alone, showed no symptoms. Cotton leaves with symptoms were taken to the laboratory to isolate the pathogen. After the development and sporulation of the fungus colony in a humid chamber (Figure 8), the fungus was isolated from the lesions produced on the inoculated leaves, and the pathogen was characterized as Corynespora cassiicola, thus concluding Koch's Postulates.

Augusto César Pereira Goulart, Fernando Mendes Lamas, Embrapa Agropecuária Oeste

Article published in issue 209 of Cultivar Grandes Culturas.