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Field Crops Cotton Symptoms, source of inoculum and management of cotton diseases. Disease Seedling Diseases (Rhizoctonia spp., Pythium spp., Fusarium spp., and other fungi) Symptoms: Loss before emergence is characterized by a rot of the seed or seedling. After emergence, affected seedlings have dark lesions on the stem, often girdling the stem and extending downward into the root system. Older plants have reddish-brown, sunken lesions near the soil line. Source of Inoculum: Some of the organisms causing seed rot and seedling diseases may be carried on the seed coat while others persist in the soil. Management: Use only high-quality seed. Plant seed only when soil temperatures at a 4inch depth reach 68 F for 3 to 4 days. Plant fungicide-treated seed to a weed-free seedbed. Plant at proper depth for soil type and weather conditions. See table on fungicides. Leaf Spots (Alternaria Symptoms: Brown to red to tan circular lesions on leaves that may occur at any stage. spp., Cercospora Lesions may be up to ¾ inch, and margins usually appear reddish to purple. As the diseases gossypina, Stemphylium progress, centers of lesions may appear sooty due to sporulation of the pathogen and spp., and other fungi) eventually fall away resulting in a “shothole” appearance of foliage. In cases of heavy infestation, lesions may overlap and cause premature defoliation resulting in decreased lint yields. Source of Inoculum: Fungi overwinter in previous crop/weed debris. Management: Plow under infected plant debris. Plant high-quality, acid-delinted, fungicide-treated seed. Avoid nutrient stresses (particularly Potassium), drought stress, other pest damage and herbicide injury. On highly susceptible varieties, foliar fungicides may reduce incidence and severity, but economic benefit is rare. Target Spot (Corynespora Symptoms: Brown to tan circular lesions on leaves usually beginning at bloom or canopy cassiicola) closure. Lesions usually will not have reddish to purple margins when found low in the canopy and will have a “bullseye” appearance. Target spot lesions are usually larger than other leaf spot lesions. The disease progresses from low in the canopy upward, and severe defoliation may occur. Source of Inoculum: The pathogen will overwinter in crop debris. Soybean is an alternative host and may harbor the pathogen. Management: Destroy debris. Some varieties may tolerate target spot better than others. Avoid rank plant growth. Scout closely prior to and after canopy closure. Scout nearby soybeans for target spot. Foliar fungicides may lower disease incidence and in some cases preserve yield. Fungicide coverage is key, and applications should be made by ground with a minimum total volume of 10 gal/A with high pressure, hollow cone or flat fan nozzles. Verticillium Wilt Symptoms: Leaf margins and between veins have pale yellow markings. Severely affected (Verticillium sp.) plants shed the young bolls. Light brown discoloration occurs in the interior woody tissue of the stem with little to no discoloration just beneath the bark. Disease occurs in cool weather with or without nematodes. Source of Inoculum: Fungus lives indefinitely in the soil. Control: Rotate with soybeans, sorghum or small grains. Fusarium Wilt Symptoms: Plants usually are stunted and may fruit early. Leaves may turn yellow, wilt and (Fusarium sp.) drop. Brown to dark-brown discoloration occurs on woody tissue just beneath the bark. It is more severe on sandy soils, during hot weather and when root-knot or reniform nematodes are present. (See root-knot nematodes below.) Source of Inoculum: Fungus lives indefinitely in soil. Nematodes, likewise, live from year to year in the soil. Management: Use of tolerant varieties. Most recommended varieties exhibit tolerance to Fusarium wilt. Under more severe conditions, use recommended nematicides. (Refer to table on nematode control in field crops.) Field Crops Cotton Symptoms, source of inoculum and management of cotton diseases. Disease Root Knot Nematodes (Meloidogyne sp.) Symptoms: Root systems are knotted or galled. Plants are stunted, slow growing and low yielding. Usually associated with a high incidence of Fusarium wilt. Damage from RKN is most severe on sandy soils. (See Fusarium wilt above.) Source of Inoculum: Root-knot nematodes live from year to year in the soil as eggs, juveniles or adults. Management: Use resistant variety. Apply nematicide. Refer to table on nematode control in field crops. Rotate to resistant variety of an alternate crop. Reniform Nematodes Symptoms: These nematodes cause severe stunting, reduced boll set and tight, locked bolls. (Rotylenchulus sp.) Root systems are restricted but not knotted. May be found in mixed to heavy soil. Source of Inoculum: Reniform nematodes live from year to year in the soil. Control: Apply nematicides. Refer to table on nematode control in field crops. There are no resistant varieties. Rotate to non-host alternate crop. Boll Rots Symptoms: There are a wide range of symptoms since there are many organisms involved (many fungi and bacteria) and many stages of boll development when damage may occur. Discolored, sunken areas may develop on the boll surface. Seed and fiber may be damaged without surface lesions on the boll. Fiber may be stained. Source of Inoculum: Organisms causing boll rots may be carried over in the soil, on crop debris or on the seed coat. Management: Avoid rank growth, and control insects during boll development. Plantgrowth regulators may be helpful in reducing rank growth and in turn, boll rot incidence. Table 2. Fungicides for the management of seedling and foliar cotton diseases. Method of Fungicide1 and Product Mode Rate3 (fl oz/A unless Target Pathogen(s) Application of Action Group2 otherwise specified) In-furrow Headline 2.08 11 4.3-11.4 Rhizoctonia solani, Pythium spp., spray Priaxor 7+11 .1-.6 fl oz/1000 row ft Fusarium spp. Quadris 2.08 11 .4-.8 fl oz/1000 row ft & Generics Reason 500 SC 11 0.45 fl oz/1000 row ft Ridomil Gold 11 1-2 Rovral 4F 4 3.4-6.9 Terramaster 4EC 14 4-8 Uniform 4+11 4.4-6.5 In-furrow Blocker 10G 14 10-20 lb/A Rhizoctonia solani, Pythium spp. granules Ridomil Gold PC GR 4+14 7-10 lb/A Ridomil Gold GR 4 1.3-2.5 lb/A Foliar spray Headline 2.08 11 6-12 Alternaria spp., Cercospora spp., Priaxor 7+11 4-8 Stemphylium spp., Ascochyta Quadris 2.08 11 6-9 spp. and Corynespora cassicola Topguard 3 7-14 Anthracnose, Boll Rots, Rust Twinline 3+11 8.5 1 Reference to commercial or trade names is made with the understanding that no discrimination is intended nor endorsement of a particular product by LSU or the LSU AgCenter is implied. 2 Mode of action groups are determined by the Fungicide Resistance Action Committee (FRAC). 3 Rates are the amount of formulation (product) per acre unless otherwise indicated. Consult product label for rates specific to row spacing, disease pressure or additional target pathogens. Field Crops Cotton Table 3. Seed treatment fungicides used to manage cotton seedling diseases. Fungicide1 and Product Target Pathogen(s)3 2 Mode of Action Group azoxystrobin 11 Pythium spp. biologicals NA Rhizoctonia solani difenconazole 3 Thielaviopsis basicola etridiazole 14 Fusarium spp. fludioxinil 12 fluopyram 7 A multitude of seed treatment options are available to cotton producers ipconazole 3 using the fungicides listed in this table and biological compounds. Seed iprodione 2 treatment fungicides are currently the preferred method of managing mancozeb M3 seedling diseases and nematodes because of convenience and efficacy. mefenoxam 4 Seed treatments with multiple modes of action are recommended to metalaxyl 4 manage a broad spectrum of seedling pathogens. myclobutanil 3 PCNB 4 pyraclostrobin 11 thiram M3 triadmenol 3 trifloxystrobin 11 1 Reference to commercial or trade names is made with the understanding that no discrimination is intended nor endorsement of a particular product by LSU or the LSU AgCenter is implied. 2 Mode of action groups are determined by the Fungicide Resistance Action Committee (FRAC). 3 Consult product label for proper targeting of pathogens. The cotton section was revised October 2015 by Dr. Trey Price.