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What are cotton seedling diseases and what causes them?
Several different, normally harmless, microscopic organisms that live on organic matter in the soil can attack cotton seedling roots in the spring. These organisms are called fungi. The ones most commonly found attacking cotton in Missouri are named Pythium, Fusarium, Rhizoctonia and Thielaviopsis. A plant may be attacked by one of these or by several at the same time. Each of these organisms causes a different disease, and the symptoms are different for each. However, they are collectively known as seedling diseases.
The organisms that cause seedling diseases are present in most soils. Once established, they remain there indefinitely. They produce structures that enable them to survive in the soil from year to year.
Seedling diseases become worse when the soil is cool and wet. These conditions do not develop in Missouri every year. Because of yearly variations in weather, the severity of cotton seedling diseases also varies. Cotton seedling diseases cause more yield loss than any other disease in Missouri (Table 1).
Table 1
Cotton yield losses (thousands of pounds of lint) due to diseases in Missouri
| Disease | Years | |||
|---|---|---|---|---|
| 1996 | 1997 | 1998 | 1999 | |
| Ascochyta blight | 0 | 5,722 | 0 | 0 |
| Boll rots | 1,455 | 286 | 342 | 0 |
| Nematodes | 1,455 | 1,430 | 855 | 1,730 |
| Seedling diseases | 4,364 | 7,152 | 1,711 | 5,767 |
| Verticillium wilt | 0 | 286 | 771 | 0 |
How do cotton seedling diseases damage cotton?
The microscopic organisms that cause seedling diseases penetrate and grow within the cotton root by secreting chemicals that dissolve the root tissue. The organisms absorb the nutrients they need for growth from the damaged root. The root damage may vary from slight injury (which the root may outgrow), to moderate injury (the plant lives but the root is permanently damaged), to seedling death. Diseased roots are unable to absorb water and nutrients as well as a healthy root, and the plant will grow more slowly. Plants with permanently damaged roots usually shed young bolls more quickly during summer drought; they mature later, yield less and produce poorer-quality lint than healthy plants.
What are the symptoms of seedling diseases?
A healthy cotton seedling root is white and firm, and the central root (taproot) is long with numerous secondary white roots emerging from the upper taproot. A stand of healthy cotton seedlings is uniform with no skips. Seedling diseases affect young plants in several ways. Dark, rotten areas (lesions) develop on infected roots (Figure 1). Seedlings may wither and die after the disease kills the root (Figure 2). The taproot may be destroyed, leaving only shallow-growing lateral roots to support the plant (Figure 3). Plants that survive infection are often weak, more susceptible to other diseases and environmental stresses, and unproductive. Sometimes seedling diseases will kill entire fields of young cotton. The most frequent result of this problem is thin, uneven stands of weakened plants (Figure 4) that grow slowly, yield poorly and have low-grade lint.
Figure 1
Rotten areas on roots damaged by seedling diseases.
Figure 2
Seedlings damaged or killed by seedling disease.
Figure 3
Shallow lateral roots left after the tap root has been destroyed by seedling disease.
Figure 4
A thin, uneven cotton stand due to seedling disease.
What can be done to prevent seedling diseases?
There is no way to eradicate the problem, but the following six steps can be taken to minimize damage:
Following these suggested procedures will give cotton farmers a better chance to produce high yields and profit. More information is available at the MU Delta Center Web site at aes.missouri.edu/delta/index.stm
MP734, new July 2000