Fusarium Diseases of Tomato: An Overview

Mike Davis, Extension Specialist, UC Davis

In 2001, tomatoes in all counties in the Central Valley and San Diego County were surveyed for Fusarium diseases. The survey results are listed in a table following this article. The range of these diseases has increased considerably in a few years.

Fusarium Wilt: Symptoms of plants suffering from Fusarium wilt include wilting and yellowing of individual branches and associated leaves. Sometimes only one branch or one side of the plant is affected, creating a yellow flag effect. Infected plants usually die. A dark brown discoloration extends far up the stem. Symptoms often first appear during fruit sizing. Fusarium wilt can greatly reduce yields in severely infested fields. The fungus overwinters and survives for many years in the soil as spores. Long distance spread is by seed, transplants, and soil on farm machinery. The disease is favored by warm weather. The fungus only infects tomato but exists as three races. Race 1 is widespread; Race 2 is common only in the Sutter Basin and south to San Joaquin County; and Race 3, once restricted to fields in the Sutter Basin, is spreading south.

Control of Fusarium wilt depends on the use of resistant tomato varieties. Resistant varieties are common for Race 1, and many are also resistant to Race 2. A few varieties are resistant to all three races. Limit the spread of infested soil by cleaning farm equipment. Avoid root knot nematode infestations since nematode feeding can overcome the plant resistance to Fusarium wilt. Rotation out of tomatoes for several years reduces inoculum level, although Fusarium is long-lived.

Fusarium Crown and Root Rot: Foliar symptoms on plants with Fusarium crown and root rot include yellowing along the margin of the oldest leaves, followed by necrosis. Dry brown lesions develop in the cortex of the tap or main lateral roots. A necrotic lesion may also develop on the surface of the stem from the soil line to 4 to 12 inches (10 to 30 cm) above it. Internally, a chocolate-brown to reddish brown discoloration extends no more than 6 to 12 inches (15 to 30 cm) above the soil line. Infected plants may be stunted and wilted.

The disease can occur in any of the California tomato-growing regions, but its occurrence is uncommon. It occasionally causes serious problems in greenhouses. In the Central Valley it causes little economical damage, but by the coast, where the cool temperatures and high humidity favor the disease, it can be a serious problem. The fungus overwinters and survives for many years in the soil as spores. Long distance spread is by transplants and in soil on farm machinery. Spores are airborne in greenhouses. The host range of the pathogen includes some legumes, cucurbits, other solanaceous plants, and more.

Fusarium Foot Rot: Foot rot of tomato caused by Fusarium solani was first reported on the Queensland Coast of Australia in 1975. In California, the disease was first reported in 1991 but was probably present since at least 1984 in Yolo County. Once confined to a few fields, the disease has spread significantly and continues to do so. Based on 2001 survey work, the incidence of foot rot in individual fields in Yolo County has been as high as 15%. Today, the disease is now found in most of the Sacramento Valley and has moved into several counties in the San Joaquin Valley.

Foot Rot Symptoms and Damage: Symptoms of foot rot include brown to reddish-brown lesions (about 1 to 2.5 cm long) of the cortex of the tap or main lateral roots with a discoloration of the stele extending 2 to 10 cm from the lesion. Root lesions usually occur within the top 30 cm of soil, and sometimes invade the crown. Affected plants are stunted with varying degrees of interveinal chlorosis, mottling, and necrotic spotting on young foliage (suggesting the production of a toxin in infected plants). Foliar symptoms may be similar to certain viruses (tomato spotted wilt or alfalfa mosaic). Flowers on symptomatic branches are often necrotic. In severely affected plants, the taproot or main lateral roots are completely girdled and the crown may be rotted. Although plants usually do not die, yields are often reduced. In Australia, all 30 commercial cultivars evaluated were susceptible. In that country the disease is apparently restricted to a few farms with poor drainage and is limited in the cool time of the year.

Control Methods: Fungicides and fumigants have been ineffective for the control of foot rot. Crop rotation is also ineffective since Fusarium solani, like many Fusarium species, has the ability to survive long-term in the soil as chlamydospores or as a saprophyte on the roots of weeds and other crop plants. Once infested, the soil may remain so for years, if not indefinitely.

Little information is available concerning foot rot epidemiology. Based on our growth chamber work, warm temperatures favor disease development (in contrast to what was reported in Australia). Like other soilborne diseases caused by soil inhabitants, there are few or no methods available for controlling the disease or eliminating the fungus from the soil. Thus, the only realistic long-term solution for managing the disease is through breeding and the development of resistant varieties. Because the disease was once limited to a few fields and disease incidence is small compared to the incidence of late blight, Phytophthora root rot, and some of the bacterial leaf spots, no concerted effort has been initiated to examine germplasm collections for resistance against this disease.

Currently, the only recommended control strategy is to limit spread of infested soil by cleaning equipment between fields.

Distribution of Fusarium diseases of tomato in California, 2001.

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