Yellow, curling leaves are often the first noticeable symptoms of several common plant diseases. It can be difficult to distinguish which disease is responsible, but that is not a huge problem for the average gardener since management techniques are similar for most these conditions. None of these diseases are easily cured, but proper culture and careful crop rotations can help prevent problems from spreading to other parts of the garden and continuing year after year.
Verticillium wilt is a soil-borne fungal disease. Visible symptoms begin with the lower leaves, which tend to yellow, then wilt and curl upwards. It is more likely to occur under cool growing conditions or in poorly drained soils. The disease enters through the root system and affects the water-conducting tissues.
Fusarium wilt begins with upper leaves. Look for a slow yellowing of the foliage, then wilting and curling of the leaves. Fusarium causes browning on internal stem tissue and cankers on plant stems near the soil line. This soil borne fungal disease is long-lived, so a five-year crop rotation is recommended to avoid reinfestation.
Plants with adequate water and nutrients are better able to resist disease. Fertilizers that are higher in potassium and lower in nitrogen are best. No proven chemical controls exist for fusarium or verticillium wilts, so crop rotation and complete removal of severely infected plants is recommended. Plants such as tomatoes, eggplants, potatoes and raspberries also can be affected and should be part of the rotation.
Several viral diseases make up the mosiac group. The wide range of symptoms can include leaves that turn yellow, curl and become mottled. Aphids, leafhoppers, thrips and other sucking insects can spread mosiac from plant to plant. Control of those insects can help prevent new cases. Tobacco mosiac virus (TMV) can also be spread by tools and even by hands that have touched affected plants.
When purchasing plants, look for varieties resistant to problem diseases. Xena Jalapeno is "TMV," which on plant labels indicates resistance to tobacco mosiac virus. The symbol for verticillium resistance is "V," and the capital letter "F" indicates fusarium resistance. Thus a variety labeled as 'Jalapeno Example VFTMV would be a variety resistant to verticillium and fusarium wilts, as well as to the tobacco mosiac virus.
Remove damp fallen leaves around the plant. Dustings of agricultural lime, flour or talc will not kill the fungus, but will help by drying the environment around the plant and making it less hospitable to fungus diseases. Sulfur dust provides some control of fungal disease but must be applied carefully. Surrounding plants that are sensitive to sulfur, such as cucumber and squash, could be harmed by the dust.