Outbreak of leaf spots caused by Corynespora cassiicola in cotton crops in Mato Grosso

Cotton target spot, caused by the fungus Corynespora cassiicola, initially reported in Mato Grosso in 2005, has acquired increasing importance. The initial symptoms of the disease are normally observed on the leaves in the lower third of the plant after the interlines close. This situation, combined with a rainy period and mild temperatures, creates an extremely favorable microclimate inside the canopy for the occurrence of infections and the development of the disease.

In the initial phase of the disease, dark spots with a yellowish halo are observed on the leaves. As they worsen, these lesions evolve and take on larger shapes, which may be irregular or circular. With development, circular lesions normally acquire a brown tone and present concentric rings of darker color, with a punctuation in the center of the lesion, resembling a target, which characterizes the name of the disease. In susceptible varieties and favorable environmental conditions, premature fall of infected leaves is observed. Defoliation and leaf lesions reduce the plant's ability to carry out photosynthesis, which may result in damage to plant development and reduced productivity.

In addition to the cotton plant, C. cassicola it infects more than 400 species of plants, including soybeans, cover crops such as sunn hemp, and also weeds such as viola string, managing to survive and multiply in various cultural remains. In the main producing regions of the cerrado, the production system is characterized by the cultivation of soybeans in the first harvest followed by cotton in the second harvest, which results in a continuity of the fungus cycle, increasing its potential for inoculum and damage to the cotton plant. Infection is favored when there is a prolonged period of leaf wetness and temperatures between 22ºC and 30°C. Temperatures below 20°C are unfavorable for the development of the fungus.

In recent harvests, leaf spots with symptoms different from those normally observed for the target spot were reported in several crops in Mato Grosso, with greater prevalence in the 2023 harvest. The circular spots observed on leaves and bracts had varied diameters, a clear center, surrounded by a dark-colored necrotic edge (Figure 1), reminiscent of the symptoms of lesions caused by Cercospora species in other cultures. In the field, lesions were observed in all thirds of the plant, with some reports of symptoms being concentrated in the upper third. Due to the symptomatological similarity and also the comparison of the symptoms observed with photos of symptoms available on the internet, it was associated and spread on social networks that these spots were caused by cercospora sp. This initial diagnosis was corroborated with the identification of typical reproductive structures of cercospora sp. in the materials analyzed and their isolation from samples received from areas with reports of the occurrence of stains. However, the greater number of reports on cultivars known to be more susceptible to target spot led us to suspect a misdiagnosis of the cause of the problem.

The intensive production system adopted in the cerrado region, with the succession of crops and the constant presence of straw on the soil surface, creates a favorable environment for the multiplication of various necrotrophic fungi, increasing the amount of inoculum of various microorganisms that are not necessarily pathogenic, which take advantage of dead plant tissue for multiplication, making their presence common on cultivated plants. Its development is favored in samples collected for analysis, if adequate processing and sending of samples for diagnosis is not adopted. Cotton leaves need to be dehydrated and packaged in a way that does not favor a high humidity environment during their journey to the laboratory, which can take days. Without adopting this procedure, opportunistic microorganisms proliferate in the material sent, leading to incorrect diagnosis of the causal agent, when this is carried out only with the detection of pathogen structures (signs) associated with the diseased tissue, isolation in pure culture and molecular analysis .

Aiming to confirm the etiology of the disease, leaves and bracts with atypical symptoms collected in commercial crops and experimental fields in Água Boa, Paranatinga, Primavera do Leste, Novo São Joaquim and Lucas do Rio Verde in Mato Grosso were evaluated (Figure 2). The cultivars sampled and which presented the greatest severity were FM978 GLTP, FM974 GLTP, FM985 GLTP, FM944GL and TMG21. Samples that were still “fresh” (one or two days after collection) or samples that were correctly herbarized before sending to the laboratory were analyzed, thus preventing the humidity of the leaf during shipping from favoring the multiplication of opportunistic fungi in associated lesions. to the diseased tissue. 

During diagnosis, the pattern of lesions and fungal structures (signs) present in the diseased tissue were first observed, then the pathogen was directly isolated from the structures observed in the different types of lesions. Additionally, molecular identification of the isolates was carried out.

80 isolates were obtained from Corynespora cassiicola (Figure 3a), three isolates from cercospora sp. and two isolates from Colletotrichum sp. The isolates from cercospora sp. It is Colletotrichum sp. were obtained from leaf regions with marked irregular necrosis, suggesting their colonization as opportunists in tissue previously killed by biotic or abiotic factors (Figure 3b).

To confirm the association between the symptoms and the isolated pathogen, the four stages of Koch's postulate were followed, which consists of: 1st stage: the possible microorganism causing the disease must be present in all cases of the disease, but absent in healthy individuals; 2nd stage: the microorganism must be isolated from the plant with symptoms, cultivated and identified; 3rd stage: the isolated microorganism must be inoculated into healthy plants and be capable of reproducing the original symptoms; 4th stage: the microorganism isolated from the inoculated plant must be identical to the originally isolated microorganism in terms of morphological/genetic characteristics.

For inoculation, six isolates of Corynespora cassiicola from different regions were selected and inoculated into cotton plants of cultivar FM985 at two stages of development. After inoculation, the plants remained in an incubation chamber for 72 hours (temperature 25ºC; RH%: 85 and without photoperiod) and were then transferred to a greenhouse. At 10 days after inoculation, all inoculated isolates were able to reproduce the symptoms observed in the samples received (Figure 4). Analysis of the fungal structures present in the lesions confirmed the presence of conidia and conidiophores typical of C. cassiicola (reproductive structures of the pathogen), demonstrating that it is the causal agent associated with the atypical symptoms.

The atypical symptomatology of the target spot observed in the field may be related to differences in cultivar resistance, similar to what was observed in soybeans with the same pathogen or differences in the physiological age of the leaves (Figure 4a). In the field, the greatest severity of this “new” symptom was observed in cultivars with greater susceptibility to target spot, and there was a constant association between the presence of typical target spot lesions in the lower third and the new symptoms in the upper thirds.

The greater occurrence of this symptomatology observed in the 22/23 harvest may be due to the occurrence of environmental conditions that favored a faster evolution of the disease, associated with failures in control programs, which normally prioritize ramularia spot, in a high pressure condition. of inoculum due to the continuous soybean-cotton succession. An additional aggravating factor is the reduced sensitivity of the pathogen to different active ingredients registered for disease control (strobilurins, carboxamides and benzimidazoles) in cotton. In this scenario, triazoles, specifically prothioconazole and multisite fungicides, play a preponderant role in disease management.

In addition to chemical control, it is important to properly manage the size of the cotton plant with regulators, avoiding creating a microclimate that is very favorable to the development of the disease. Another important measure is the destruction of the cotton ratoon to reduce the survival of the pathogen and the continuous increase in the inoculum favored by the soybean-cotton succession. Crop rotation could also contribute to reducing inoculum present in production areas. However, there are economic and technical limitations to its adoption, such as the wide range of hosts C. cassiicola and its ability to multiply in cultural remains.

Pathogenicity assessments of the remaining isolated fungi are ongoing. The resistance of the isolates obtained to different site-specific fungicides used in cotton cultivation is also being evaluated to verify whether this outbreak may also be related to variation in the sensitivity of C. cassiicola to triazole fungicides currently used in control programs.

By David Sacon, Valeria C. Holtman e Cesar M. de Oliveira (UFV); Maurício Silva Stefanelo e Guilherme Almeida Ohl (Ceres Consultoria Agronomica); Sérgio H. Brommonschenkel (UFV)

Article published in issue 294 of Cultivar Grandes Culturas Magazine