Lettuce drop is caused by two fungi, Sclerotinia sclerotiorum and S. minor, and is a common disease of lettuce in many cool, moist climates. Infection by S. sclerotiorum occurs when sclerotia (dormant resting structures) germinate when the soil surface is continuously wet for at least two weeks. The germinating sclerotia form small, tan, cup-shaped structures called apothecia, which release millions of airborne ascospores that infect lettuce. S. minor, however, rarely produces apothecia. S. minor germinates eruptively, producing masses of hyphae that can directly infect roots, stems, and senescent leaves. Both fungi can survive in soil as sclerotia for up to 8 to 10 years, and have broad host ranges.
White mold occurs in two phases. The first phase occurs immediately after thinning on a low percentage of plants. The second phase, when the majority of infections occur, is at or near crop maturity. Initial symptoms appear as a wilting of the outermost layers of leaves, giving the plant a stressed appearance. As disease progresses, other leaf layers wilt and eventually the entire head wilts. A soft, watery decay follows on both above- and belowground plant parts. A snowy white mycelium and black, irregularly shaped sclerotia become apparent as disease progresses. Disease losses vary depending on cropping history and weather conditions, but disease losses as great as 70% are not uncommon.
Contans is a commercial formulation of a fungus pathogenic to S. sclerotiorum sclerotia, and may reduce white mold incidence and/or severity if applied to soil over many years.
Avoid excessive irrigation and fertilization that leads to dense, lush canopies favorable for white mold. Promote air movement within the canopy by planting cultivars with smaller, upright architecture, planting rows parallel to prevailing wind direction, and planting on wider row spacing. These practices will reduce the duration of leaf and soil wetness and may help to reduce white mold in semi-arid production areas. If possible, use drip irrigation or schedule irrigations to prevent continuous leaf and soil wetness. Deeply incorporate crop debris to bury sclerotia.
Chemical controls are most effective when combined with cultural control strategies.
Common/Trade Name |
Product per Acre |
Application Frequency (days) |
Remarks |
Boscalid |
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Endura |
8-11 oz |
7-14 days |
Maximum of 22 oz or 2 applications per season; 14 day PHI |
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Cyprodinil / Fludioxonil |
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Switch 62.5WG |
11-14 oz |
14 days |
First application at thinning and 14 days later; maximum of 56 oz per season; 7 day PHI |
Iprodione |
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Rovral 50 |
1.5-2 lb/10 gal |
10 days |
Maximum of 3 applications; 14 days PHI |
Nitroaniline |
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Botran 75 |
2.3 lb/100 gal |
At thinning |
Maximum of 5.3 pounds per season |
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality. |
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Categories: Lettuce, Disease, White Mold
Date: 04/01/2007