Fusarium oxysporum - infected seedlings are stunted. Older leaves droop and curve downward, others yellow. Vascular tissue turns brown and the bases of stems enlarge. The plant wilts and eventually dies.
Older plants will first begin to yellow, often just on one side. Or leaflets will yellow on one side and not the other. Plants begin to wilt during the hottest part of the day. This progressively worsens and the plants will eventually die. Infection of older plants generally occurs at time of blossoming to fruit maturation. Vascular tissue will turn brown and this is often used for identification. Cut a section of stem from the lower part of the plant on the diagonal, vascular tissue often will be disclored. Similar in appearance to this picture of vascular discoloration from bacterial wilt.
Fusarium is a warm weather disease. It is most prevalent on acid, sandy soils (though certainly not limited too). The pathogen is soil-borne and can persist for several years. Three separate races of the pathogen have been identified, Races 1,2 and 3. Race 1 is the most widely distributed. Race 2 was first reported in Ohio in the 1940's and did not become a problem until 1961 in Florida. It then became prevalent worldwide. Race 3 was first reported in Brazil in 1966. Race 3 has since become a problem in commercial production areas of the US (Fla & Ca). Race 3 has been showing up in South and North Carolina fields. It is suspected that it moved up from Florida on transplants, machinery or migrant workers shoes. Resistance to Fusarium race 1 and 2 is multigenic (several genes confer resistance). This makes it harder for the pathogen to overcome the resistance. So far, resistance found to race 3 is monogenic (one gene). There is concern that the race 3 of the pathogen could eventually overcome this resistance. (It's much easier too overcome one instead of many). Identification of races can only be done in lab or possibly by comparing cultivars against know sources of resistance. This costs $, and takes time, so most home gardeners should seek varieties with resistance to as many races as possible.
The pathogen enters the plants through wounds in roots growing in infested soil. The following conditions favor it's development:
Fusarium can be spread on seed, stakes, soil, infected transplants, equipment, tools and shoes/clothing. Long distance spread is through seed, transplants or infected soil. Fusarium can even spread in the wind with dust particles. It is not considered to be spread by insects.
Control of fusarium is achieved by utilizing resistant cultivars. Commercial growers often fumigate or steam areas to be planted. This is not practical or economically possible for most home gardeners. Solarization can be effective and a biological control agent (a microbe called Trichoderm 3 [Russian]) has been shown to be effective. Raising the soil to Ph 6.5 -7.0 and using nitrate sources of fertilizers can help. Research has shown that this step alone has the same effect on yield as fumigation in a 5.5 pH soil. Irrigation by flooding is discouraged as it can spread the disease. Ditch water and pond water can be infected as well. One should excersise caution when cultivating as not to damage roots which would allow for entry of the pathogen.
Raising ones own transplants in a sterile disease free media and using disease free seeds can go along way as well. Heavy mulches could help by lowering the soil temperature and slowing the growth of the pathogen.
A 5 to 7 year crop rotation does not eliminate the pathogen but can reduce losses. It is recommended that you rotate your Solanaceous crops (peppers, potatoes, eggplant, flowering tobacco, petunias).
A quick check of selected cultivars with resistance to at least two races includes:
Just remember, resistance does not necessarily mean it won't get the disease, rather it is less likely to be as much of a problem. Hang in there "Fusarium Fighters"