Target spot in cotton crops

By Luiz Gonzaga Chitarra, Wirton Macedo Coutinho and Fabiano José Perina (Embrapa Cotton)

04.10.2024 | 16:32 (UTC -3)
Figure 1 - Cotton leaves infected by the fungus , the etiological agent of target spot
Figure 1 - Cotton leaves infected by the fungus Corynespora cassiicola, etiological agent of target spot

Cotton target spot, caused by the fungus Corynespora cassiicola (Berk. & Curtis) Wei, was first reported in Brazil in 1995, in the state of Mato Grosso. Until 2005, due to its low incidence, the pathogen did not cause significant damage to the crop. After this period, sporadic outbreaks of the disease were reported in cotton-producing areas, mainly in the Cerrado of Mato Grosso. Currently, it is a widely distributed disease that causes damage to the crop in the main cotton-producing regions of Brazil and is most severe in the states of MT, BA, GO and MS.

Corynespora cassiicola (Berk. & Curtis) Wei, is a polyphagous fungus that attacks about 530 plant species from 380 plant genera. In cotton, this fungus is capable of infecting leaves, flowers, stems and roots of plants, destroying the host tissue in order to utilize its nutrients. The initial symptoms of target spot on cotton leaves are characterized by small circular spots of brown or purplish coloration.

As the disease develops, these spots become irregular lesions with concentric rings of chlorotic edges that can vary in color from dark green to light brown, with a dark brown center. Abscission of part of the leaf tissue may or may not occur. The number and size of lesions on the leaves may vary. The lesions range from 0,5 to 20 mm in diameter, depending on the evolution of the pathogen in the leaf tissue. In severe cases, the lesions coalesce, causing severe necrosis, followed by premature senescence and death of the leaf. Rapid defoliation is caused by the acceleration of senescence, which makes the leaf yellow in a few days. Lesions may also occur on cotyledons, bracts and young bolls.

Environmental conditions favorable to the development of the pathogen in cotton crops are cloudy days, prolonged and uninterrupted periods of rain, with intense wetness on the leaf surface and temperatures between 25 ºC and 30 ºC. In cotton, the disease progresses more rapidly after the closure of the plant canopy, conditions that are favorable to the development of the pathogen due to the high shading of the leaves in the lower and middle layers of the crop, causing intense defoliation under severe infections. Other factors that contribute to the rapid development of the disease are inadequate management of growth regulators, smaller spacing between cotton rows, tall plant populations and susceptible cotton genotypes.

In a recent study conducted by technicians from Embrapa Cotton, the severity of target spot was evaluated in 36 cotton genotypes in the field, in the 2021/2022 harvest. It was observed that there are differences in susceptibility between the genotypes evaluated (Figure 2).

Figure 2 - severity (%) of target spot in cotton genotypes - Ipiranga do Norte (MT), 2021/22 harvest
Figure 2 - severity (%) of target spot in cotton genotypes - Ipiranga do Norte (MT), 2021/22 harvest

In another study, carried out to evaluate the susceptibility of cotton genotypes to target spot, it was also found that all genotypes evaluated, to a greater or lesser degree, showed susceptibility to the disease (Figure 3). In this same study, a linear correlation between the severity of target spot and the plant population was observed, demonstrating that, as the population of cotton plants per hectare increased, there was a significant increase in the severity of the disease (Figure 3).

Figure 3 – linear regression graph for the severity of target spot in cotton, caused by in six different cotton genotypes, as a function of the stand (plants per linear meter), at a spacing of 76 cm - adapted from Perina et al., 2019
Figure 3 – Linear regression plot for the severity of target spot in cotton, caused by Corynespora cassiicola in six different cotton genotypes, depending on the stand (plants per linear meter), at a spacing of 76 cm - adapted from Perina et al., 2019

The dispersion of C. cassiicola in the main cotton-producing areas of Brazil, it occurs mainly through rain and wind. Wind is the main route of dispersion of the pathogen over short distances. Chlamydospores, the resistant structures of the fungus present in the soil, can also be disseminated from infested areas to pathogen-free areas through agricultural implements, rainwater and irrigation. Regarding the transmission of the pathogen via seeds, although there are several studies that prove the contamination of cotton seeds by the pathogen, there is no evidence of transmissibility of C. cassiicola from seed to adult cotton plant.

The spread of this fungus in cotton crops in the Brazilian Cerrado is probably related to the production models currently used in the Cerrado areas, especially the crop succession system. The continuous soybean-cotton succession system, common in many agricultural regions of the Cerrado, can cause an increase in the pathogen population and, consequently, lead to a decrease in cotton productivity. This system is potentially favorable to the development and spread of the pathogen, since both soybean and cotton are susceptible to the disease.

Furthermore, due to its wide range of hosts, the spread and distribution of the pathogen in the main cotton-producing regions in the Cerrado are favored by the increase in planting areas associated with inadequate elimination of crop residues, the soil management system, the succession of planting of susceptible varieties and/or host species cultivated in the same area and climatic conditions favorable to the development of the disease.

It is worth highlighting the importance of controlling plant height using growth regulators, using lower planting densities and greater spacing between rows. All of this is aimed at providing greater aeration in the lower third of the plants. These cultural practices are also effective in controlling the disease.

Just like the cultural methods described above, the use of chemical fungicides is essential for controlling the causal agent of cotton spot. Thirteen fungicides are currently available on the market and are registered with the Ministry of Agriculture, Livestock and Food Supply (MAPA). The chemical groups that make up these fungicides are carboxamides, dithiocarbamates, triazolinthiones, triazoles, isophthalonitriles and strobilurins. These chemical groups are also part of the fungicides that are used to control the etiological agents of ramularia spot, the main disease of the aerial part of the crop, and for which an average of 4 to 8 applications are made during the harvest. These applications indirectly contribute to the control of cotton spot and are essential in the integrated management of the disease.

By Luiz Gonzaga Chitarra, Wirton Macedo Coutinho e Fabiano José Perina (Embrapa Cotton)

Article published in issue 287 of Cultivar Grandes Culturas Magazine

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