Managing White Mold in Potato: Symptoms, Control, and Fungicide Solutions
February 27, 2024
White mold is an important disease of potato and many other broad-leaf crops.
The white mold pathogen overwinters in the soil as sclerotia, which can remain viable in the soil for up to five years.
White mold management programs include the integration of cultural practices and the application of fungicides.
Luna® Pro fungicide and Luna Tranquility® fungicide can help to manage white mold and several other key fungal diseases of potato.
White mold, also called Sclerotinia stem rot, is a disease caused by the fungal pathogen Sclerotinia sclerotiorum. The white mold pathogen has a host range of over 400 plant species, including many broad-leaf crop and weed species. Yield losses resulting from white mold range from negligible to 100%, and the disease can become a serious and persistent problem in some locations.4 Potato varieties show some variation in susceptibility; however, all widely used commercial varieties are considered susceptible to white mold.5,6
Symptoms
White mold symptoms initially develop on the potato plants’ lower leaves and stems, typically ten to fourteen days after row closure. Water-soaked lesions with distinct margins develop, often in the stem branch points or where stems are in contact with the soil. The lesions become covered with a white, cottony mat of fungal growth. 1,2,3 When severe, the lesions can expand to the point where they girdle the stem, resulting in wilting of the foliage, and plants can be killed. With time, the lesions will turn tan to beige to bleached and dried out, with the tissue becoming papery. Stems can be hollowed out as they decay from within and eventually split open. 3,4,7 The fungus can spread to nearby stems and leaves if conditions in the canopy are moist.
Black, hard, irregular-shaped bodies, called sclerotia, form on the mycelial mats on and within the stem. The sclerotia initials are round, white masses of fungal tissue. Over time, the sclerotia develop a hard, black exterior and a white interior. Sclerotia are ¼ to ½ inch in diameter and can be over an inch long.2,3,4
Disease Cycle and Conditions
The pathogen overwinters as sclerotia in the soil or infested crop residue. The pathogen is spread within fields when sclerotia are moved during cultivation, in moving water, and in windblown crop debris. Sclerotia can also be transported to previously non-infested fields in soil and debris on contaminated equipment. However, there is little or no plant-to-plant spread of white mold during the growing season, with infections initiated from the overwintered sclerotia. The sclerotia can remain viable in the soil for up to five years.1,3,4
In the spring and summer, sclerotia germinate and produce a mushroom-like structure called an apothecium. Germination is favored by cool temperatures (50° to 70°F), high relative humidity (95 to 100%), and several days of moist soil conditions. Therefore, germination is more likely to occur after canopies have closed to shade soil surfaces and reduce air circulation. Typically, only sclerotia in the upper two inches of the soil will germinate. Sclerotia deeper in the soil can remain viable for up to five years and germinate when they are brought up near the soil surface.1,2,3,4
The apothecia that form on germinating sclerotia are small (4 to 8 mm diameter), beige to pink, mushroom-like bodies on the soil surface. The apothecia produce microscopic spores, called ascospores, which are disseminated by air currents. The peak of spore production and release often occurs when the potato plants are in full bloom. The spores can spread the disease to other plants within the field or to neighboring fields. The spores land on and colonize plant tissues starting to senesce, such as older flower petals. When colonized flower petals are dropped from the flower, they may land on the lower stems and get lodged in branch points. The pathogen can then use the flower petal as a food source to provide the energy needed to infect healthy plant tissues, such as stems. Ascospores cannot infect healthy tissue directly.1,3,4
Management
White mold in potato is best managed with an integrated program of cultural practices and fungicide applications. Currently the application of fungicides is the primary management tactic. The choice of fungicide, application method, and timing of application are important in managing white mold on potato. Fungicide treatments should be initiated when plants reach the full bloom stage to help prevent the flower petals from becoming infected by ascospores. The number of applications needed will depend on the products selected, the length of the season, and the length of the flowering period. Applications should deposit fungicides deep into the canopy to help protect flowers as well as the potential infection sites, such as stem branch points, where colonized flower petals may land.2,3,4 Always read and follow label directions and restrictions.
Several fungicides are labeled for the control of white mold on potato, most as foliar applications, but some as in-furrow applications.2,3,7,8 Fungicides that contain the active ingredient fluopyram, including Luna® Pro fungicide and Luna Tranquility® fungicide, have been shown to protect potatoes from white mold and other key foliar fungal diseases, including early blight. Fluopyram is a systemic fungicide with uniform uptake by the plant tissues, enabling the product to enter and protect buds, blooms, and new tissues. Both Luna® Pro and Luna Tranquility® are combination products. Luna® Pro combines fluopyram with prothioconazole (FRAC group 3) that provide protection throughout the plant, protecting against both foliar and soilborne pathogens. Luna Tranquility® combines fluopyram (FRAC group 7) with pyrimethanil (FRAC group 9) providing an effective combination of preventative and curative activity.
Applications of both Luna products can be repeated at 7- to 14-day intervals, but there are restrictions for the total amount of each active ingredient that can be applied each season.
Other fungicides recommended for controlling white mold include products containing the active ingredients boscalid, fludioxonil, fluazinam, iprodione, penthiopyrad, or thiophanate-methyl.2,3,7,8
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Cultural practices are used to create environmental conditions that are less favorable for infection and to lower inoculum levels in the field.3,4 The tillage of crop residues promptly after harvest can help limit the formation of overwintering sclerotia and help prevent the inoculum buildup in the field. Long-term crop rotation combined with the management of weed hosts can help lower the levels of white mold inoculum over time. Rotate to non-host crops, including corn and small grains. Longer rotations away from potatoes and other crop hosts will be needed with higher inoculum levels.3,4,7,8 Tillage can bury sclerotia deep enough so that they will not germinate the following year. However, subsequent cultivations can bring these sclerotia back to the upper soil levels, where they can germinate. Flooding soils for three to six weeks has been shown to reduce the viability of sclerotia in the soil.1,2
When possible, avoid planting potatoes in fields with a history of severe white mold. Also, avoid planting in areas that are shaded, have poor air circulation, or have poor soil drainage. Choose row spacings that provide an acceptable balance of adequate airflow in the canopy with high levels of tuber production. Avoid the over-application of nitrogen fertilizer that can result in excessive canopy growth. Irrigation timings and amounts should be designed to reduce the hours of leaf wetness. Irrigating early in the morning can give time for foliage to dry before nightfall.2,3,4,7,8
Sources:
1 Icochea, T. 1981. White Mold. In W. J. Hooker (Editor) Compendium of Potato Diseases, ppg. 48-50. The American Phytopathological Society.
2 Nuñez, J., Aegerter, B., Davis, R. 2019. White mold. Potato Management Guidelines, UC IPM.
3 Kirk, W., and Hao, J. 2011. White mold or Sclerotinia stem rot in potatoes. Michigan State University Extension.
https://www.canr.msu.edu/news/white_mold_or_sclerotinia_stem_rot_in_potatoes
4 Heffer Link, V., and K. B. Johnson. 2007. White Mold. The Plant Health Instructor. DOI: 10.1094/PHI-I-2007-0809-01. Updated 2012.
5 Miller, J., Taysom, T., Suarez, C., Anderson, D., and Miller, T. 2021. Interaction between fungicide programs and variety on foliar potato diseases in southern Idaho, 2020. Plant Disease Management Report No. 15:V096.
6 Wharton, P. and Wood, E. 2013. White mold of potatoes. University of Idaho Extension. CIS 1200.
7 Phillips, B., Nair, A., Bergefurd, B., Egel, D., Ingwell, L., and Meyers, S. 2023. Midwest vegetable production guide for commercial growers 2023. https://mwveguide.org/.
8 Reiners, S., Bihn, E., Curtis, P., Helms, M., McGrath, M., Nault, B., Seaman, A., and Sosnoskie, L. 2023 Cornell Integrated Crop and Pest Management Guidelines for Commercial Vegetable Production. Cornell Cooperative Extension.
The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about vegetable production. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with vegetable crops. BAYER GROUP DOES NOT WARRANT THE ACCURACY OF ANY INFORMATION OR TECHNICAL ADVICE PROVIDED HEREIN AND DISCLAIMS ALL LIABILITY FOR ANY CLAIM INVOLVING SUCH INFORMATION OR ADVICE.
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