| [Image: Picture of chlorotic cotton plants] |
| [Image: infected cotton stem showing discoloration] |
| [Image: Picture of necrotic cotton leaves] |
Symptoms
Seedlings can be infected and killed by Fusarium wilt, and early-season seedling death is often confused with seedling diseases. Initial symptoms on older plants include wilting and/or yellowing (chlorosis) of the foliage. Wilt most often occurs gradually, but a sudden increase in wilting often is seen after rain. Chlorosis starts at the margins of the leaf and between the main veins, and eventually turns necrotic. Infected leaves die and fall from the plant. The vascular system of infected plants is brown to black and is easily visible below the bark when the stem is cut. Severely infected plants are stunted and may die.
Yield losses can severe in infested fields. The time of infection determines the extent of yield reduction. Plants infected early usually die before producing any bolls, and plants infected after the onset of flowering often survive to produce a few bolls. These plants mature earlier and open bolls sooner than uninfected plants.
Causal Agent and Disease Development
Fusarium wilt, caused by the fungus Fusarium oxysporum f. sp. vasinfectum, is a serious disease of cotton. The pathogen survives for long periods in the soil and infects plants through the roots.
Fusarium wilt often forms a disease complex with the root-knot nematode that increases the severity of wilt. While it is important to remember that these pathogens are capable of creating yield losses separately, their combined action can be much more severe than with either alone.
Control
Fusarium wilt is difficult to control. There are many cotton varieties with moderate resistance and a few with high levels of resistance. Consult current variety lists to identify resistant varieties. Crop rotation is often recommended, but it is effective only for long rotations because of the ability of the fungus to survive in the soil for long periods in the absence of cotton.
Often controlling the root-knot nematode is the best approach to controlling Fusarium wilt. Practices recommended for controlling the root-knot nematode generally also reduce the incidence of Fusarium wilt. There are cotton varieties with moderate resistance to the Fusarium wilt and root-knot nematode disease complex, but these may still suffer damage from the nematode.