Photos by Debbie Roos, Agricultural Extension Agent.
This basil is suffering from Fusarium wilt, a devastating disease caused by a soilborne pathogenic fungus. According to NCSU’s Basil Horticultural Informational Leaflet, Fusarium oxysporum f. sp. basilicum was first discovered in the U.S. in 1991 and identified in N.C. in 1992. Plants infected with this disease usually grow normally until they are six to twelve inches tall, then they become stunted and suddenly wilt. Initial symptoms usually include brown streaks on the stems, discoloration of the internal stem tissue, a shepherd’s crook appearance of stems, and sudden leaf drop (see photos below for examples of all these symptoms). Interestingly, only sweet basil is affected. Some of the specialty basils, such as lemon basil and purple basil, show some resistance to the disease.
The disease is introduced into fields, hydroponic systems, and greenhouse culture primarily through contaminated seed. Growers should only buy basil seed that has been tested for the fusarium wilt fungus. Currently, these tests involve growing out a large number of seed and looking for disease symptoms. This does not guarantee that the seed will be free of infection, but it greatly reduces the risk. If it is not possible to obtain tested seed, the seed should be soaked in cold water for four hours followed by a heat treatment of 20 minutes in 133-136° F water. Seed germination rates will probably be reduced by the hot water treatment, so a germination test should be conducted on a small lot of the treated seed to determine how much seeding rates need to be adjusted. Also, the hot water treatment causes a sticky layer to develop on the outer surface of seed making it difficult to handle. Some seed companies also sell resistant varieties.
Once a field has become infested with the fusarium wilt pathogen, infective propagules may persist in the soil for 8-12 years. During that time, growers should avoid growing sweet basil or members of the mint family. Mints will not exhibit symptoms of the disease but may carry over the inoculum from year to year. There are currently no products registered to help control this disease.
This page last updated January 15, 2010.