Reviewed October 1993
Contents
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Cotton sales are very important to many nation's economies, including the United States. Sales can help balance trade with cotton-buying countries.
Problems such as diseases can interfere with cotton production and can therefore upset the producing countries' economies. The situation is the same in the U.S. Although the effects of cotton diseases on the U.S. economy are not as dramatic as in some other countries, crop damage due to severe disease can seriously effect an individual producer's profit. Fortunately, most cotton diseases can be managed.
To manage disease and nematode problems, an accurate diagnosis is essential. This guide should help you to identify problems caused by cotton diseases and nematodes and to decide which management strategies to use.
Whenever possible, consult one of us at the
There are several strategies for managing cotton disease and nematode problems in cotton. The three most important are: rotate crops, plant resistant varieties, and plant in warm, well-drained soil. An integrated approach that uses all of these methods usually is the most effective and profitable.
Improved varieties immune to all or most cotton diseases do not exist. However, newer cotton varieties often have improved levels of resistance to root-knot nematode, Fusarium Wilt and Verticillium Wilt (Table 1). Farmers should choose varieties based on MU yield trials in their area and resistance to locally significant diseases.
Table 1
Resistance of some cotton varieties to some diseases
(Information on private varieties was submitted to us by the companies. An "X" indicates that the company feels the variety has some resistance to the disease.)
| Variety | Fusarium wilt | Verticillium wilt | Root-knot nematode |
|---|---|---|---|
| Delcott | X | X | X |
| DPL 20 | X | X | X |
| DPL 50 | X | X | X |
| DPL 51 | X | X | X |
| DPL 5690 | X | X | X |
| DPL 5415 | X | X | X |
| DES 119 | X | X | X |
| HyPerformer HS 23 | X | ||
| HyPerformer HS 46 | X | X | |
| Stoneville 453 | X | ||
| Stoneville 506 | X | ||
| Stoneville 907 | X | ||
| Coker 130 | X | X | |
| Sure-Grow 1001 | X |
Rotating cotton with corn or certain soybean varieties will help manage several cotton diseases. For example, the number of root-knot nematodes in the soil declines when Forrest, a root-knot nematode resistant soybean variety, is planted. The population of this nematode will fall below the cotton damage threshold after planting Forrest for a few years.
Certain growth regulators cause the plant to grow more open, which increases air movement, reduces canopy humidity and thus reduces boll rot. Boll rot is most prevalent during warm, humid, rainy weather, especially when bolls are injured by insect feeding. To avoid boll rot, plants should be protected from insects and managed to avoid rank growth, which can increase humidity in the canopy.
Seed treatments and fungicides applied to the soil at planting can be very helpful in managing cotton seedling disease. Most commercially sold cotton seed is treated with a fungicide. Farmers should have a fungicide applied to all seed they have saved.
Fungicide seed treatments help protect the seed against rot, but will not protect the seedling from all disease. A fungicide applied to the soil at planting will provide additional protection against seedling disease. Soil-applied fungicides may be placed in the furrow or in a "T" band over the row. They can be applied as a spray or granule. A current listing of registered soil-applied fungicides for cotton is provided in this publication.
Table 2 provides a brief description of the major diseases and nematodes found in Missouri cotton. It also provides management strategies to follow when they occur. Pesticides listed for management of specific diseases were labeled for those uses as of Aug. 1, 1992.
Table 2
Cotton disease and nematode management strategies
| Disease and cause | Symptoms | Chemical control | Alternative control and remarks |
|---|---|---|---|
| Seedling disease | |||
| Rhizoctonia, Fusarium and Pythium spp. | Slow emergence, thin stands, lesions on stems at soil line, dry or soft rot of roots, lodged seedlings | Apply one of the following in-furrow fungicides during planting: Terraclor Super X (10 to 15 pounds per acre = 12 to 18 ounces per 1,000 feet row); Terraclor Super X EC (2 to 3 quarts per acre); Terraclor EC + Ridomil 2E (2 to 3 quarts + 4 to 8 ounces per acre); Ridomil PC (7 to 10 pounds per acre = 8.5 to 12 ounces per 1,000 foot row. | Plant only high-quality (80 percent+ germination) fungicide treated seed. Plant in warm, well-drained soil. Avoid crop stress due to a lack of fertilizer, excess water or excess herbicides. Plant on raised beds. |
| Boll rots | |||
| Small brown to black lesions on bolls expand and the entire boll eventually may become blackened and dry. | None. | Prolonged periods of high humidity or water on the boll surface is necessary for infection. Rank growth promotes boll rot. Avoid practices that result in a rank dense canopy. Use a growth regulator such as Pix to reduce plant height. | |
| Bacterial blight | |||
| Xanthomonas campestris pv. malvacearum. | Small, light green, angular lesions on leaves which turn dark brown. Large numbers of lesions may darken large areas of a leaf. Infected leaves defoliate prematurely. Symptoms rarely occur on bolls or stems in southeast Missouri. | None. | Most varieties have some resistance. Crop rotation will control this problem. |
| Leaf spots | |||
| Alternaria macrospora. | Small, brown, circular lesions on leaves enlarging to 0.4 inches. Old lesions have gray centers which may fall out. | None. | These leaf spot diseases are of minor importance in Missouri and specific controls are not recommended. They appear more frequently on stressed plants. |
| Cercospora gossypina. | Small purple lesions on leaves expand to light brown lesions surrounded by narrow purple margin. These old lesions may be 0.7 inches in diameter. | None. | Same as for Alternaria macrospora. |
| Ascochyta gossypii. | Small (2 mm) brown lesions on leaves that enlarge into irregular dead areas surrounded by a dark brown border. Lesion centers may crack and fall out. | None. | Same as for Alternaria macrospora. |
| Alternaria. | Small purple spots develop on dying leaves. These lesions resemble those caused by Alternaria macrospora. | ||
| Fusarium wilt | |||
| Fusarium oxysporum f. sp. vasinfectum | Symptoms usually appear first on oldest leaves at bloom. Areas of leaf margin wilt then turn yellow. The entire leaf eventually may wilt and turn yellow. Leaves defoliate prematurely. In cross-section, a brown layer of tissue appears just below the bark of the lower stem. Bolls may open prematurely. These symptoms are difficult to distinguish from Verticillium wilt. | None. | Most varieties are somewhat resistant. Maintain soil pH at 6.0 to 6.5, and use recommended rates of fertilizers. |
| Verticillium wilt | |||
| Once called fusarium | |||
| Root-knot nematode | |||
| Meloidogyne incognita. | Above-ground symptoms depend on the level of infestation. At high levels, plants may be stunted, yellowing and unthrifty in appearance. Plants may wilt in the heat of the day and recover at night. Yields are reduced. Symptoms may be confused with mineral deficiency, drought, herbicide injury or other soil-borne diseases. Symptoms occur in oval patches or spaced erratically through the field. Roots have small to large galls and may be excessively branched. Roots usually are discolored. | The following products are labelled for soil fumigation: | Rotation with small grains or resistant soybean cultivars. Few resistant cotton cultivars are available. |
| Telone II (52 to 106 ounces per 1000 foot row). Do not plant for 1 to 2 weeks. | |||
| Vapam (75 to 100 gallons per acre=1.5 to 2 pints per 100 square feet). Do not plant for 2 to 3 weeks. | |||
| The following chemicals are labelled for soil treatment: | |||
| Temik 15G (3.5 to 10 pounds per acre = 4.5 to 12 ounces per 1000 foot row) | |||
| Nemacur 15G (6.5 to 14.7 per acre = 8 to 18 ounces per 1000 foot row). | |||
| Reniform nematode | |||
| Rotylenchulus reniformis. | Light green color appears by the third or fourth leaf stage. Leaf margins may become purple. Other above-ground symptoms same as root knot. | Same as for root-knot nematodes. | No resistant cotton varieties. Same as for root-knot. |
| Lance nematode | |||
| Hoplolaimus galeatus. | Above-ground symptoms same as root-knot. Plant may defoliate following drought. Small, brownish-yellow lesions on roots. | Same as for root-knot nematodes. | No resistant cotton varieties. Same as for root-knot nematodes. |
G4261, reviewed October 1993