Watch out for White Mold on Your Tomato Crop

white mold of tomato

White mold on tomato also is known as timber rot. Photo courtesy of Maine.gov

Sclerotinia white mold on tomato is caused by Sclerotinia sclerotiorum. S. sclerotiorum has a wide host range, attacking more than 170 species. This disease affects a number of economically important vegetable crops including beans, cabbage, lettuce, and pepper.

In tomato, the disease is also known as timber rot. Primary infections usually occur on flowers and succulent tissues. The spores go through a saprophytic growth stage on senescent flowers before initiating further infection. Initial infection of the pathogen is on tissues within the plant canopy, often near the base of the stem at the soil line.

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Identification
Pale or dark-brown, water-soaked lesions form on flowers and at stem joints where senescent flower petals have fallen. Bleached areas and watery, soft rots form on the stems and leaf axils, and then wet, fluffy white mold develops inside and outside the plant tissue. The soft, watery rots on the stems eventually become dry and brittle, which leads to girdling. The pathogen also can attack at the base of the stem causing the plants to wilt and die.

As infected tissue decays, hard, black, irregularly shaped resting structures called sclerotia form on the inside and outside of decaying tissue. Stems are frequently hollowed out by the fungus leaving a papery shell to cover numerous sclerotia.

Survival and Spread
Sclerotinia sclerotiorum can remain dormant in the soil for five years or more as hard, black sclerotia and require a conditioning period of cool temperatures before they can germinate and form apothecia — cup-shaped fruiting bodies. The pale, brownish-yellow apothecia form just above the soil line and produce ascospores that spread through moving water, wind, plant debris, and workers.

The pathogen favors temperatures from 59°F to 70°F, and nighttime temperatures of 60°F. Sixteen to 72 hours of continuous wetness and high humidity is favorable for spore infection. Once the disease cycle is complete, spores are not produced again until the next season.

Management Methods
Effective management of white mold requires an integrated disease management approach. The disease is controlled primarily through the use of cultural practices and foliar fungicides.

Scouting is important for early detection once plants begin flowering.

Cultural practices, such as destruction of infected plant debris, eradication of weed hosts, and crop rotation with non-susceptible hosts like corn, will help reduce disease in subsequent plantings.

Good fertility management to prevent excessive canopy development and staking to improve aeration can aid in reducing problems with white mold.

Contans WG (Coniothyrium minitans, Bayer) is a biological product that limits the seasonal carryover of sclerotia and must be applied prior to and following the cropping season. Preventative fungicide applications will help prevent infections.

Consult UF/IFAS recommendations for currently labeled fungicides for sclerotinia control in Florida tomato.