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Upland cotton (Gossypium hirsutum) is considered to be the most important fiber crop grown in the world, and the United States is ranked among the top five producing countries. Cotton is produced in 64% of Alabama’s counties ranging from the north, central, and southern regions of the state, and the value of the cotton lint and seed produced in Alabama during the last decade has averaged $171,371,000 annually. Seedling disease is a major obstacle to cotton production in Alabama. Seedling diseases reduce yields by 6% annually, which translates to losses of approximately $10,282,200 each year. Severe seedling stand losses force producers to replant their crops, which involves the added cost of supplemental seed, insecticides, herbicides, and application expenses. Time also is a limiting factor because a producer can replant only if enough time is left in the growing season to produce a crop. Pathogenic fungi are the primary causes of seedling disease; however, little is known about the types of fungi involved throughout the state and there are conflicting results in the literature pertaining to fungal diseases associated with cotton production in Alabama. An AAES study was instituted to catalogue the various fungi affecting cotton health in Alabama. Plant samples were collected using a randomized systematic sampling pattern from fields representing the major cotton growing regions of the state. The fields were located at the Gulf Coast Research and Extension Center in Baldwin County (South Alabama), the Wiregrass Research and Extension Center in Henry County (Southeast Alabama), the E.V. Smith Research Center in Macon County (Central Alabama), and the Tennessee Valley Research and Extension Center located in Limestone County (North Alabama).
Cotton fields in each region were sequentially sampled at two weeks after planting (seedling stage), first bloom, full bloom, and maturity. Plant material was taken from the tap roots, petiole, and boll of each plant and cultured for fungi. Fungal colonies were immediately matched to their respective species based on reproductive morphology and colony characteristics, or subcultured for further identification (Figure 1). The number of colonies of each fungus per plate was recorded for each anatomical location, sample date, and test site. Relative recovery was calculated by dividing the number of colonies of individual fungi by the total number of all fungal colonies recovered from cotton tissue of each replicate and expressed as a percentage (see Figure 2). A total of nine Fusarium species were isolated from cotton seedlings across all locations. Fusarium species isolated from cotton samples collected from the Tennessee Valley and southeast Alabama included Fusarium equiseti, F. lateritium, F. oxysporum,, F. semitectum, and F. solani. Fusarium species isolated from cotton samples collected from Central Alabama included those previously mentioned plus F. moniliforme. Fusaria isolated from cotton samples collected from the Gulf Coast added the species of F. longipes, F. proliferatum, and F. subglutinans, none of which had not previously been reported on cotton. The relative recovery of Fusarium spp. was highest at seedling stage for every location except Central Alabama. Pythium ultimum and P. aphanidermatum were recovered from cotton seedling root tissue at every location except the Gulf Coast. Rhizoctonia solani was isolated from cotton root, stem, and petiole tissues collected across all locations. It was recovered at maturity from every location except the Tennessee Valley. Thielaviopsis basicola was isolated from cotton roots at seedling and/or first bloom collected from every location except Southeast Alabama. It also was recovered from roots as late as full bloom in Central Alabama. The growing season in 2000 was extremely dry in Alabama. This factor alone may have resulted in low disease severity and incidence of fungal pathogens. Therefore, this project will be conducted over several years and at multiple locations in which soil factors and weather condition will be analyzed throughout the growing season. Change in weather patterns, development of new cropping systems, pesticide use, new cultivars, and other management methods all impact the mycoflora living in agricultural soils. Future attempts to control numerous fungal diseases such as cotton seedling disease will be more successful with a better understanding of pathogens and the variables that influence their development.
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