Ascochyta or Wet Weather Blight – (fungus – Ascochyta gossypii): The disease is prevalent in most cotton producing areas of the state. Both seedlings and older plants are susceptible, but younger cotton is more seriously injured. An entire stand may be lost as a result of the fungus attacking the hypocotyl and killing the plant. Serious outbreaks of the disease may follow extended rainy periods with serious defoliation occurring. The damage is generally spotty and many plants recover when dry, warmer weather returns. The disease occurs on the leaves, stems and branches. First, small, round, reddish-brown spots with a dark brown border appear on the leaves. Later, the center of the lesions become ashy in color and may fall out. The lesions often occur at the base of the petiole. Defoliation may result from large lesions that coalesce. On stalks and branches, the lesions are dark brown, elongated and slightly sunken cankers. Under conditions favorable for the disease, the lesions may completely encircle the stem or branch and kill the plant above the lesions. The fungus may be seed-borne but it primarily survives in the soil on the infected plant residue. Use of acid delinted seed and suitable seed treatment fungicides will minimize carry-over. Plowing under plant residues and crop rotation also aid in reduction of the disease.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Root rot appears suddenly, starting in early summer. It causes rapid wilting, followed by death of the plants within a few days (Figure 3). Usually, the leaves of the plant are not shed, but remain attached. The disease kills plants in circular areas ranging from a few square yards to an acre or more in size. The root system of affected plants decay. Scraping the taproot reveals a darkened, reddish to wine-colored stain. If examined soon after death, the stems will be near normal color internally. Vascular streaking is not present as in the wilt disease. Fine, light brown strands of fungal threads (rhizomorphs) are usually found on the roots. Under moist conditions spore mats may appear on the soil surface near diseased plants. These are 2-12 inches in diameter, first snow-white and cottony, later tan and powdery. Controls include management and cultural practices as no chemical treatment has been found that economically controls cotton root rot. Deep plowing approximately 12 inches with mold board plow, planting early maturing varieties to escape disease, addition of organic matter or green manure and crop rotation have proven to be the most effective control methods. To achieve maximum control, an integrated program involving all practices is recommended.
Root Knot (nematode – Meloidogyne incognita acrita): The symptoms caused by root knot vary from slight plant stunting to death in areas of severe infestation. Skippy stands, particularly in distinct areas of a field, are characteristic. In skip-row cotton, where current rows are laid out perpendicular to the previous year’s row, stunted cotton outlining the old rows may be noticed. Root knots are small and should be checked on plants dug with a shovel and not those pulled by hand, since pulling plants often results in just the tap root being extracted. Control root knot nematodes by: rotation with sorghum or small grains, and use of in-furrow nematacides.
Nematodes Other Than Root Knot (nematodes – lesion: Pratylenchus sp., spiral: Helicotylenchus sp., lance: Hoplolaimus sp., sting: Benlonolaimus sp., stunt: Tylenchorhynchus sp., reniform: Rotylenchus reniformis): Of these nematodes, reniform is the most important in cotton production in Texas. It has been found in very small numbers in three counties in the High Plains, but it is most prevalent in the Rio Grande Valley area.
Verticillium Wilt (fungus –Verticillium albo-atrum): Young plants infected with Verticillium wilt show yellow leaves and stunting, and often die. Following the seedling stage, older plants exhibit a chlorotic mottling on the leaf margins and between the major veins. (Figure 4) Plants attacked during later stages of growth display a mottling on the lower leaves first, later progressing toward the top of the plant as the season progresses. Often a single branch shows symptoms in the early stages of disease. Yellow progresses inward, followed by brown, and the leaf finally dies. Severely affected plants shed all their leaves and most of their young bolls. These plants may survive throughout the growing season and send out young sprouts or shoots from the base of the plant. Verticillium wilt is difficult to distinguish from Fusarium wilt. Leaf symptoms are similar and the internal tissues of the stems are discolored in both diseases. The only reliable way to distinguish the wilts is through a laboratory isolation of the fungus. The fungus causing Verticillium wilt can survive in the soil as small, dark resting bodies called sclerotia. The sclerotia can withstand adverse environmental conditions. Susceptible plants growing in Verticillium infested soil may not be severely attacked if environmental conditions are not suitable for fungal growth. The disease is more prevalent during periods of cool, wet weather. The fungus is not transmitted in seed, but can be introduced in the field by infested gin trash and burrs. The disease is more common on heavier soils than on light sandy soils. Control measures include avoiding excess nitrogen, crop rotation, shallow cultivation to avoid root pruning, avoiding excess irrigation, and planting resistant varieties.
Bacterial Blight (bacterium – Xanthomonas campestris pv. malvacearum): Also known as angular leaf spot, vein blight, black arm and boll rot, depending on the portion of the plant infected. This organism affects all above ground parts of the cotton plant during any stage of its growth. Angular spots first appear on the leaves as water-soaked areas; the spots later turn dark brown to black and are covered with a glazed film. Often the surrounding tissue becomes yellow, giving a halo effect. Leaf spots are limited in size by leaf veins, which result in an angular shape. Infected leaves shed from seedlings and older plants. Occasionally, a black, water-soaked area occurs along a large vein in a leaf. Spots on bolls appear as round, water-soaked areas, but later turn dark brown or black. Spotted bolls may fail to open and lint may be discolored with a yellow stain. Before boll rot is evident, dark, irregularly shaped spots can be found on bracts surrounding the lower portion of the boll. Black spots or cankers may occur on the stems or branches (black arm) causing girdling and death of some branches. Overall, leaf spots and defoliation are the most noticeable symptoms and they are most likely to occur during or following rainy periods. Control measures depend largely on eliminating sources of infection and growing resistant or tolerant varieties. Since the bacterium overwinters in crop residue, plowing under the stubble immediately after harvest and practicing crop rotation will help reduce inoculum in the field. Acid delinting of cottonseed has eliminated carry-over. Most varieties have good resistance to bacterial blight.
Fusarium Wilt (fungus – Fusarium oxysporum f. sp. vasinfectum): Fusarium wilt is more prevalent in the lighter-textured acid soils of Texas. Unlike Verticillium wilt, seeds from diseased plants can become infected and serve to spread the fungus. The fungus may attack cotton seedlings, but the disease usually appears when the plants are more mature. Affected plants are first darker green and stunted, followed by yellowing of the leaves and loss of foliage. First, symptoms appear on lower leaves around the time of first flower. The leaf margins wilt, turn yellow, then brown, moving inward. Infected plants fruit earlier than normal with smaller bolls that open prematurely. A diagonal cut across the stem will reveal vascular discoloration just beneath the bark extending down the tap root. Wilting occurs rapidly following a rain preceded by a dry spell. Soils in which Fusarium wilt occurs also favor root knot nematodes and the two are often found together. Reniform nematodes (which occur mostly in the lower Rio Grande Valley) also predispose the plant to attack by the fungus. Control of nematodes is of major importance in reducing Fusarium wilt. Cultural practices effective for reducing Fusarium wilt losses include avoiding seed from infested fields in humid climates, rotation with non-susceptible crops, use of in-furrow nematacides, and varieties with resistance.
Rust (fungus – Puccinia cacabata): True cotton rust is quite distinct from the “rust” caused by potash deficiency. Cotton rust is confined to the Trans-Pecos area of West Texas, where it may reduce yields as much as 50 percent. Rust first appears as small, yellowish spots or pustules on leaves, bracts, green bolls, and stems. These enlarge, developing orange to reddish centers. Later, large orange pustules appear on the lower leaf surface and discharge orange spores (aeciospores). Several lesions on a leaf may cause it to shed. The rust lesions will also weaken stalks, stems and petioles, causing breakage on these parts. Broken stalks are more difficult to cultivate and harvest mechanically. The aeciospores released on the cotton do not reinfect cotton, but are windblown to wild gramma grass, which serves as an alternate host for the fungus. (Figure 7). A rainfall of « inch or more, followed by 12 – 18 hours of high humidity, is needed in June or early July for disease development. The only effective means of control is with an application of a mancozeb foliar fungicide before the cotton is infected. This is the only labeled fungicide, but it can not control the fungus after infection has occurred.
Virus Diseases: Several virus diseases have been described on cotton but are only of minor importance in Texas
Boll Rots (fungi and bacteria – caused by many parasitic and saprophytic organisms including: Colletotrichum sp. and Xanthomonas sp.): Symptoms may vary depending on the organisms involved. Generally, the first symptoms of boll rot are small, round, water-soaked spots on the bolls which may enlarge and become sunken and dark in color. Eternal symptoms may be absent but complete destruction of seed and fiber will occur. Infected bolls may be reduced by avoiding practices that promote rank growth, keeping cotton free of weeds and grasses, and skip- row planting. Heavy insect injury increases boll rot incidence. Boll rot is a problem of South and Central Texas. It is rarely seen on the High Plains.
Minor Leaf Spots (fungi – Alternaria sp., Cercospora sp., Rhizoctonia sp., Stemphyllium sp.): Two or more leaf spots may occur at the same time. Symptoms are varied, but generally these organisms cause circular concentric lesions similar to a target spot . These foliar diseases tend to be more prevalent at crop maturity and during periods of high humidity. Preventive control measures are most effective and include seed treatment, good management practices, and destruction of plant residue after harvest. Some varieties are more susceptible to late season leaf spot fungi.
Seedling Disease Complex (fungi – Rhizoctonia solani, Fusarium sp., Pythium sp., Thielaviopsis basicola): Seedling disease is caused by a complex of soil fungi which may occur separately or in combinations. These fungi are Pythium, Fusarium, Rhizoctonia, and Thielaviopsis. Symptoms include decay of the seed before germination, decay of the seedling before emergence, girdling of the emerged seedling at or near the soil surface, and rotting of root tips. Any one of these fungi or several working together can produce these symptoms.
Several practices reduce losses to seedling disease. These include using rotation, quality seed, timely planting and the use of fungicides. Producers can expect increased disease control as they increase the number of control practices used.
Rotation is the first line of defense against seedling disease. Following cotton with cotton will increase populations of seedling disease fungi in the soil. Fields should be rotated out of cotton for at least two years. Sometimes, problem fields can be identified and rotated out of cotton for a longer period of time.
The second line of defense is seed quality. Quality seed will emerge from the soil more quickly and develop secondary roots faster, therefore being vulnerable to fungal infection for a shorter period of time. Seed should be tested using the Cool-Warm Vigor Index (CWVI). This test is available at the Texas Department of Agriculture Seed Laboratory in Lubbock. Earliest plantings which have the greatest potential for seedling disease infection should be planted with seed rated excellent or at least good on the Cool-Warm Vigor Index.
Timely planting is the third step in reducing seedling disease loss. Unusually warm weather early in the season often entices growers to plant earlier than they should. Even good quality seed should not be planted until the 10-day average soil temperature at the eight-inch depth is 65o F.
The most underrated seedling disease control practice is the use of fungicides to protect the seed and seedling. Most are applied by the seed supplier. However, hopper box treatments are also available. The advantage of hopper box treatments is that the decision whether to use them can be made the day of planting. Hopper box treatments in recent years have not out-performed the better seed treatments in replicated tests. The highest level of input for seedling disease control involves in-furrow fungicides. Cost of in-furrows is generally higher, in the $10.00 per acre range, compared to approximately $2.00 per acre for two of the better seed treatments in combination. In-furrows also have not out-performed the better seed treatments in tests on the High Plains.
There are a large number of fungicides labeled as seed treatments for seedling disease control, but only a few of them are used to any great extend in West Texas. These include Captain, Vitavax, Vitavax-PCNB, Apron, Nu-Flow M, Demosan and Baytan. Using combination treatments of some of the new systemic fungicides will provide a higher level of disease control. Combinations of two or three systemic fungicides in which each material is effective on a different seedling disease fungus provides much better control than just Captain. These double fungicide seed treatments cost approximately $5.00 – $7.00 per bag. This will be a $2.00 – $5.00 per acre cost at the 20 lbs. per acre planting rate recommended for the South Plains.