Molecular Microbiology Research 2024, Vol.14, No.4, 181-187 http://microbescipublisher.com/index.php/mmr 182 Alternaria leaf spot disease in cotton cultivation to promote the development of more effective and sustainable disease management practices, ultimately improving the productivity and profitability of cotton cultivation. 2 Pathogen Identification and Biology 2.1 Characteristics of Alternaria species Alternaria species are known for their distinct morphological features and pathogenic capabilities. The colonies of Alternaria alternata, a common species causing leaf spot in cotton, typically appear olive green with distinct white margins and relatively small spores compared to other large-spored species (Figure 1) (Zhu et al., 2019). The conidia of A. alternata are obclavate to elliptical, with both cross and longitudinal septa, measuring 16.2 µm to 30.5 µm long and 7.5 µm to 10.6 µm wide (He et al., 2022). These morphological characteristics are consistent with the genus Alternaria and are crucial for the identification of the pathogen in infected plant tissues. Figure 1 Morphological characteristics of Alternaria colonies isolated from cotton. A: Colony morphology of Alternaria colonies on PDA after 7 days; B and C: sporulation pattern of Alternaria colonies on PDA after 7 days (Adopted from Zhu et al., 2019) 2.2 Life cycle and infection process The life cycle of Alternaria species involves several stages, starting from spore germination to the development of necrotic lesions on the host plant. The spores of A. alternata germinate under favorable environmental conditions, such as temperatures between 20 °C and 30 °C, and initiate infection by penetrating the host tissue. The pathogen produces conidia that are dispersed by wind, water, or mechanical means, leading to secondary infections. The infection process involves the formation of necrotic lesions, which expand and coalesce, causing significant damage to the plant tissues (Prasad et al., 2017; Tao et al., 2021). Infected leaves exhibit symptoms such as circular, pinhead necrotic lesions surrounded by a purple halo, which eventually lead to premature defoliation. 2.3 Factors influencing disease development Several factors influence the development and severity of Alternaria leaf spot in cotton. Environmental conditions, such as temperature and humidity, play a crucial role in the proliferation of the pathogen. Optimal growth of A. alternata occurs at temperatures between 20 °C and 30 °C, with high humidity favoring spore germination and infection. Stressed cotton plants, due to adverse environmental conditions like chilling stress, are more susceptible to infection, leading to premature leaf senescence and increased disease severity (Liu et al., 2018). The presence of secondary hosts, such as Prosopis farcta, can also contribute to the spread and persistence of the pathogen in the field (Olmez et al., 2023). Furthermore, genetic factors, including the resistance of different cotton genotypes, influence the incidence and severity of the disease. Studies have shown significant genotypic variations in disease severity among different cotton cultivars, with some genotypes exhibiting lower disease severity indices (Zhu et al., 2018). 3 Symptoms and Field Diagnosis 3.1 Visual symptoms on cotton leaves Alternaria leaf spot in cotton is characterized by distinct visual symptoms that can be observed in the field. Initially, small, circular, necrotic lesions appear on the leaves, often surrounded by a purple halo. As the disease
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