Molecular Microbiology Research 2024, Vol.14, No.4, 181-187 http://microbescipublisher.com/index.php/mmr 183 progresses, these lesions expand and coalesce, forming larger, irregularly shaped necrotic areas that can cover significant portions of the leaf surface. In severe cases, the infection can spread to other parts of the plant, including bracts, petioles, stems, and bolls, leading to premature defoliation and significant yield loss (Olmez et al., 2023). The visual symptoms are similar to those described in other crops affected by Alternaria species, such as muskmelon and chaste-tree, where small, brown, necrotic spots with concentric rings are common (Ahmed et al., 2021; Wang et al., 2023). 3.2 Laboratory methods for confirming diagnosis To confirm the diagnosis of Alternaria leaf spot, laboratory methods are employed. Infected leaf samples are collected and surface-sterilized before being plated on potato dextrose agar (PDA) or other suitable media. The samples are incubated under controlled conditions to promote fungal growth. Morphological examination of the fungal colonies under a microscope reveals characteristic features of Alternaria species, such as dark olive-green colonies and conidia with transverse and longitudinal septa (Liu et al., 2022; Yu et al., 2022). For precise identification, molecular techniques are used. DNA is extracted from the fungal isolates, and specific regions such as the internal transcribed spacer (ITS) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes are amplified using polymerase chain reaction (PCR). The amplified products are sequenced and compared with reference sequences in genetic databases to confirm the identity of the pathogen. This approach has been successfully used to identify Alternaria alternata as the causal agent of leaf spot in various crops, including cotton, muskmelon, and chaste-tree. Pathogenicity tests, where healthy plants are inoculated with the fungal isolates and observed for symptom development, further confirm the diagnosis. 4 Epidemiology and Spread 4.1 Environmental conditions favoring disease spread Alternaria leaf spot in cotton is significantly influenced by environmental conditions. Studies have shown that the disease thrives in warm and humid climates. For instance, in southern New Mexico, the prevalence and incidence of Alternaria leaf spot have increased due to favorable environmental conditions, particularly temperatures ranging from 20 °C to 30 °C, which are optimal for the growth of A. alternata (Nguyen and Le, 2023). Similarly, in Türkiye, the disease was observed to cause severe defoliation and desiccation of cotton plants during the mid to late season, which coincides with warm summer temperatures. These findings suggest that warm temperatures and high humidity are critical factors that favor the spread of Alternaria leaf spot in cotton. 4.2 Seasonal dynamics of disease incidence The incidence of Alternaria leaf spot in cotton varies with the season. In New Mexico, the disease was evaluated in October and November, during the late growth stage of the plants, where a high disease incidence of up to 100% was recorded in most fields (Ramirez-Villacis et al., 2023). This seasonal pattern indicates that the disease is more prevalent during the late growing season when environmental conditions are conducive to the pathogen's growth. Similarly, in Türkiye, the disease symptoms were observed in the summers of 2021 and 2022, affecting a significant portion of the cotton fields (Mohamed et al., 2019). These observations highlight the importance of monitoring and managing the disease during specific times of the year to mitigate its impact on cotton production. 4.3 Role of inoculum sources The sources of inoculum play a crucial role in the epidemiology and spread of Alternaria leaf spot in cotton. In Türkiye, a secondary host, Prosopis farcta, was identified in the field where symptoms of the pathogen were observed, indicating that alternative hosts can serve as reservoirs for the inoculum (Kumar and Raghavendra, 2023; Gao, 2024). This finding underscores the importance of managing not only the primary crop but also potential secondary hosts to reduce the spread of the disease. The study in New Mexico identified A. alternata as the causal agent of the disease, with isolates characterized based on morphological features and PCR analysis (Rajarammohan et al., 2019). Understanding the specific pathogens involved and their sources of inoculum is essential for developing effective control strategies.
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