Cotton Diseases

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Arnet, J.D.; Bertrand, P.; Crawford, J.; Ellis, HC; Lambert, W.; McGlohon, N.; Suber, F.; Thompson, S.; Womack, H.; Brown, E.A.; Evans, B.R. Insect and Disease Identification Guide for IPM in the Southeast. The University of Georgia. Cooperative Extension Service Bulletin 849. September 1981. 59p.

Contents

Boll Rot (fungi Diplodia gossyina, Fusarium spp.)

The soilborne fungi which cause boll rot survive on plant debris. Diplodia spp. attacks lower bolls near maturity in warm, humid conditions resulting from a thick growth canopy and high moisture from rains and dews. Bolls turn dark brown and as rot progresses, the black smut-like fungus growth which can be easily rubbed off covers the boll surface. Fusarium spp. are capable of infecting bolls about 35 days or older. Diseased bolls become dark brown with a white to salmon-pink overgrowth of the fungus. Fusarium spp., Diplodia spp. and other fungi have been associated with a progressive basal type of rot where bracts are infected first, followed by invasion through nectaries and base of boll. Fusarium spp. and D. gossypina infecting at the cracking stage may cause "tight-lock."

Nematodes (root-knot Meloidogyne incognita, sting Belonalaimus longicaudatus)

Root knot larvae penetrate roots and as females develop and lay eggs, swollen knots or galls are produced. The knots are smaller and less prominent than on more susceptible host plants. Careful digging is necessary to prevent stripping off infected roots, thus incorrect diagnosis. Infestations are usually greater in sandier soils and occur in irregular shaped areas in fields. Infected plants show a stunted growth, foliage discoloration and are often infected by the Fusarium wilt fungus. The sting nematode is more common on extremely sandy soils, primarily in the Coastal plains. This destructive pest severely reduces root development and plant growth. Secondary roots and right angle branching are reduced when the tap-root is attacked during the early growth stages. Sting injury is often followed by Fusarium wilt fungus infection.

Fusarium Wilt (fungus Fusarium oxysporium f. sp. vasinfectum)

The Fusarium wilt fungus usually penetrates through root injury caused by nematodes. Infected areas are irregular in shape and size. Water-conducting stem tissues turn brown and become inactive, resulting in wilted foliage. Leaves turn yellow between veins and eventually shed as plants die to leave bare stems. Diagnosis is confirmed by splitting the stem to reveal dark brown, vascular discoloration and streaking characteristic of wilt.

Seedling Diseases (fungi Rhizoctonia solani, Pythium spp.)

Seed rots, root rots and damping-off or "soreshin" are various stages of seedling disease complex. Rhizoctonia sp. infection is usually characterized by dark, reddish-brown to black rotted zones on the stem at or just below soil line. Pythium spp. causes similar injury with a water soaked lesion at the soil line.

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