Tomato Spotted Wilt

The symptoms of tomato spotted wilt in pepper vary depending on the stage of growth that the plant is infected, the cultivar, co-infections with other viruses, and other factors such as environmental conditions. Certain symptoms of TSWV infection—the spotting, bronzing, and necrosis of leaves and the ringspots on fruit—are fairly typical.

Seedling Infection

Plants infected at the early stages of growth (e.g., as transplants) are stunted. Leaves will be stunted with necrotic spots or rings. Severely infected plants may die.

Early Infections in the Field

One of the earliest initial symptoms is a bronze appearance on infected leaves, along with a drooping or wilting of the infected plant. This is associated with development of necrotic spots on leaves, which may include vein necrosis, as well as necrosis of the stems and petioles. Eventually the entire plant becomes stunted and may show a drooped or wilted appearance; necrosis becomes more pronounced on infected leaves, petioles, and stems. Developing green fruit will be bumpy and show diagnostic spots and concentric rings that are initially pale or yellow but may become necrotic. Ripe fruit are distorted and often show extensive necrotic rings or etching.

Late Infections in the Field

When plants are infected later in development, only a part of the plant may develop symptoms, whereas the rest of the plant will remain healthy (this is because of the inability of the virus to move into mature parts of the plant). The initial symptoms in leaves include curling, pale green to yellow discoloration, and purpling of the leaves. Fruits on such infected shoots may become bumpy, deformed, and often develop spots, ringspots, and necrosis.

Tomato spotted wilt virus is transmitted by various species of thrips, including the western flower thrips, Frankliniella occidentalis, the onion thrips, Thrips tabaci, and the chili thrips, Scirtothrips dorsalis. Tomato spotted wilt virus also infects the thrips vector. This virus is not seedborne and it is not spread by contact; it is only spread from plant to plant by thrips.

Nymphs that acquire the virus by feeding on infected plants will retain the ability to transmit it for the remainder of their lives. Tomato spotted wilt virus cannot be passed from infected females through the eggs.

TSWV has a wide host range, and can infect hundreds of species of plants, including monocots and dicots. These plants include crops, ornamentals, and weeds. However, it is important to emphasize that all plant hosts are not equally important in contributing to the development of tomato spotted wilt disease in crop plants. It is only the species of plants that are infected by TSWV and on which the thrips can complete their entire life cycle that play an important role in the disease cycle. In California, the key field crop hosts include tomato, pepper, radicchio, and lettuce. Important weed hosts include little mallow (Malva parviflora), sowthistle (Sonchus oleraceus), and prickly lettuce (Lactuca serriola), among others.

Effective management of tomato spotted wilt in areas where it is known to occur in California requires an integrated pest management (IPM) approach that targets the thrips vector and the virus.

This IPM strategy can be divided into three parts:

Before the Growing Season

• Use virus- and thrips-free transplants (ideally from transplant houses that monitor for thrips and inspect for disease).
• Manage thrips populations on transplants if necessary (for more information, see THRIPS ).
• Practice good weed management in and around fields to be planted with pepper.

Note that TSWV-resistant pepper cultivars suitable for use in California are not yet available.

During the Growing Season

• Avoid planting new fields near older fields (especially those fields confirmed to have TSWV infection).
• Monitor fields for the presence of thrips and manage populations (see THRIPS for more information).
• Monitor for TSWV using a number of tests including the enzyme-linked immunosorbent assay (ELISA) and immunostrip tests that are based on antibodies that recognize TSWV proteins and the polymerase chain reaction test (PCR) that detects the virus genetic material. The immunostrip is a rapid result test for plant viruses that involves the use of 'dip-sticks' that are put in bags with sap prepared from plant samples; the results are obtained in 5-10 minutes. TSWV immunostrips and buffer bags are commercially available from companies such as AgDia (www.agdia.com) and EnviroLogix (www.envirologix.com).
• Consider roguing plants infected at the seedling stage.
• Practice good weed management in and around fields.

After the Growing Season

• Sanitation is very important; it is important to promptly remove and destroy old crops and volunteers after harvest (e.g., plowing and physical removal). Ideally this practice is followed on a regional basis.
• Control weeds and any volunteers on fallow fields or unused land nearby where tomato crops will be planted.