White Mold
Beginner
Scientific Name
Sclerotinia sclerotiorum
Identification
Stems: The fungus produces a dense white mycelium on the surface of stems. As infection progresses, lesions may girdle the stems, causing the leaves to wilt above the infected area infected stems. When the weather is dry, lesions dry out and turn tan or bleached white in color. Severely affected stems become hollow and break easily.
Sclerotia are produced either inside or on infected stems.
Leaves: Leaf infections usually start in spots where infected blossoms fell or at the base of the leaflets. Young lesions have a water-soaked appearance and are pale green and of irregular shape. Later a white fungal growth develops on lesions.
Often Confused With
Late
blight
Period of Activity
High humidity favours disease development. Thus, the disease usually develops when the rows are closed.
Scouting Notes
- Scouting should start before the rows close.
- Fields should be monitored at least twice a week.
- When walking fields, stop at many sites as possible and check plants at random.
- Always check low spots and wettest areas of fields
Thresholds
No thresholds are established.
Advanced
Scientific Name
Sclerotinia sclerotiorum
Identification
Stems: The fungus produces a dense white mycelium on the surface of stems. As infection progresses, lesions may girdle the stems, causing the leaves to wilt above the infected area infected stems. When the weather is dry, lesions dry out and turn tan or bleached white in color. Severely affected stems become hollow and break easily.
Sclerotia are produced either inside or on infected stems.
Leaves: Leaf infections usually start in spots where infected blossoms fell or at the base of the leaflets. Young lesions have a water-soaked appearance and are pale green and of irregular shape. Later a white fungal growth develops on lesions.
Often Confused With
Late
blight
Biology
The white mold fungus persists in the soil as sclerotia. Sclerotia are black, hard, irregular resting structures that may survive in soil for at least 3 years. They range in size from 1 mm to over 1 cm in diameter.
Sclerotia near the soil surface germinate to form structures called apothecia that release spores. Spores infect leaves and blossoms when plant surfaces are wet for an extended period of time. Infected petals that fall and stick to stems or leaves cause more infections. If the weather is dry, sclerotia germinate by producing mycelium that penetrates the stems at ground level. Low to moderate temperatures and high relative humidity favour the development of white mold.
The disease is more prevalent in the wettest parts of fields. Usually first infections are noticeable after vine closure when free moisture remains longer on leaves and stems.
The fungus attacks many other vegetables, legume crops and weeds.
Disease incidence is higher if potatoes are planted after susceptible hosts such as beans or soybeans. Other factors that increase disease incidence are heavy canopy growth, high relative humidity and free moisture on plants for extended periods of time.
Period of Activity
High humidity favours disease development. Thus, the disease usually develops when the rows are closed.
Scouting Notes
- Scouting should start before the rows close.
- Fields should be monitored at least twice a week.
- When walking fields, stop at many sites as possible and check plants at random.
- Always check low spots and wettest areas of fields
Thresholds
No thresholds are established.
Management Notes
- Avoid excess nitrogen which causes heavy canopy growth that creates conditions favorable for white mold.
- Match irrigation with crop needs. Do not over-irrigate.
Cultural practices, such as eradication of weed hosts and crop rotation with non-susceptible hosts like corn, or weak hosts such as small grains should reduce the incidence of white mold in subsequent potato plantings. White mold sclerotia can survive for several years in the soil. Thus, it may be necessary to grow non-susceptible crops in long rotations to reduce inoculum levels significantly.
