Symptoms and Signs
The Fusarium wilt fungus infects plants through the rootlets, invading the xylem and eventually extending throughout the plant. Individual branches and associated leaves on plants infected with Fusarium become yellow and wilt. Sometimes only one branch or one side of the plant is affected, creating a yellow flag effect. Infected plants usually die. A dark brown vascular discoloration extends far up the stem. Symptoms often first appear during fruit sizing.
Comments on the Disease
Symptoms of Fusarium and Verticillium wilts are similar and may require culturing the fungus in a laboratory for positive identification. Fusarium wilt can greatly reduce yields in fields with a high incidence of Fusarium. The fungus overwinters and survives for many years in the soil as spores and on the outer surface of other plants, such as weeds and other crops, without causing them harm. 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 in the Sacramento Valley and in the northern San Joaquin Valley; and Race 3 is in the Sacramento Valley and spreading into the San Joaquin Valley.
Use 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 because 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.