White Mold Disease

Published: May 17, 2021

Figure 1 White mold (aka timber rot) of tomato. Photo: Peng Tian

The MU Plant Diagnostic Clinic recently received an alfalfa sample and a tomato sample. Both of them were diagnosed with white mold disease caused by Sclerotinia sclerotiorum. The white mold disease on tomato is also called timber rot while on alfalfa, it is called Sclerotinia crown and stem rot. This disease can also affect soybean, clover and many vergetables. In the cool and moist condition, the infections are primarily on the crown and stem tissues of the plant. As the disease progresses, white mold spreads inside and outside of the stem, causing stem and crown rot. When humidity and temperature become favorable for this disease, black and hard structures with irregular shapes, also known as sclerotia, are produced among the whilte mold. They are the overwinter survival structures that can stay in the soil for many years, making it very difficult to control this disease.

The black and hard sclerotia are very characterisitic for white mold disease caused by Sclerotinia sclerotiorum, although there are other diseases that are also caused by Sclerotinia spp. The southern blight disease of tomato is caused by Sclerotium rolfsii, which is very easy to be confused with white mold disease. In cotrast with the mophology of sclerotia of white mold, southern blight yields many tan to reddish-brown, spherical sclerotia (1 to 2 mm in diameter) and they are very similar in both shape and size. In addition to alfalfa, Sclerotinia sclerotiorum is a very common disease for soybean. Another Sclerotinia crown and stem rot of alfalfa is caused by Sclerotinia trifolium, which is the one most commonly identified on alfalfa.

Figure 2 Black sclerotia produced on the selective media. Photo: Peng Tian

Disease management:

1. Crop rotation is not very effective since the sclerotia can survive in the soil for extended period of time and the disease has a broad host range.
2. Sanitation by removal the infected plants carefully is important for small farm growers or home gardeners.
3. This disease favors high humidity and cool weather, thus improvement of air circulation and prevention of water splash are helpful in reducing the spreading of this disease.
4. Weeds control is helpful to control this disease since it can open more space for the plants to increase the air flow and one the other hand, susceptible weeds can become inoculum sources of new infection.
5. There are limited number of resistant/tolerant cultivars for some species of plants available in the market.
6. Fungicides labeled for this diseases are available for several plants especially the row crops but they may not be cost-effective. Please follow the label instruction and restrictions when using them.

References:

1. Sclerotinia Crown and Stem Rot on Alfalfa (AKA: White Mold), Jaime Cummings of NYS Integrated Pest Management, and Janice Degni of Cornell Cooperative Extension, Cornell Field Crops (https://blogs.cornell.edu/ccefieldcropnews/2019/05/24/sclerotinia-crown-and-stem-rot-on-alfalfa-aka-white-mold/)
2. Southern Blight of Tomato and Pepper, Inga Meadows, Amanda Scherer, Michelle Henson, NC State University Extension (https://content.ces.ncsu.edu/southern-blight-of-vegetable-crops)
3. White Mold on Tomato, Paula Flynn, Iowa State Unviersity Extension and Outreach (https://hortnews.extension.iastate.edu/2006/6-28/tomato_mold.html)
4. White mold on tomatoes, Margaret McGrath, Vegetable Pathology - Long Island Horticultural Research & Extension Center (https://blogs.cornell.edu/livegpath/gallery/tomato/white-mold-on-tomatoes/)

For appropriate diagnosis, the MU Plant Diagnostic Clinic can help you confirm if your plant has this disease. We encourage you to visit our website (https://extension.missouri.edu/programs/plant-diagnostic-clinic) and review submission guidelines before submitting your sample. If possible, you may take photos and send them to plantclinic@missouri.edu.

We just uploaded a new webinar about sample submission guidelines on YouTube to help you submit your sample step by step.
Please click here: https://www.youtube.com/watch?v=4dUcYKKFwaI

For sample submission and fee payment, you can either:

1) Visit our new online submission system at https://extension.missouri.edu/services/plant-disease-sample. Fill out the submission form online using your computer or mobile device and make payment online securely with a credit card.

2) Download the submission form at https://extension.missouri.edu/programs/plant-diagnostic-clinic/sample-submission. Fill it out and send to us together with your sample and payment. Check or money order. No cash please.

Contact Information:

University of Missouri-Plant Diagnostic Clinic
28 Mumford Hall
Columbia, MO 65211
Phone: 573-882-3019
Email: plantclinic@missouri.edu