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Elevated risk for white mold (Sclerotinia stem rot) in pulse crops

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White mold/Sclerotinia Stem Rot of pulse crops

How to recognize white mold disease

Symptoms typically start to appear after the plant canopy closes. At the field level, you may observe patches of dead plants (Figure 1). On individual plants, characteristic symptoms include bleached stems and leaves, and under high humidity, a very characteristic white fluffy mold (Figure 2). Dark sclerotia also form on plant tissues. Sclerotia are the survival structures of the pathogen (Figure 3). Early infections can consist of a grey-brown rotted lesion (soft rot), possibly with brown ooze coming from the tissue. Wilting of tissue occurs within days of infection.

Disease Cycle

White mold is caused by the fungal pathogen Scleortinia sclerotiorum. The fungus is very long-lived in the soil and is favored by high humidity and temperatures of 50-78° F. Highly dense crop canopies high moisture from repeated rain events are conducive for disease development.

The fungus survives as sclerotia, which are small, round masses of hyphae covered by a black rind. If you break them open, they are white/beige. The sclerotia lay on the surface of the soil or in the top 2” and can either germinate directly and infect the plant or form an inverted cup mushroom called ‘apothecia’ that ejects spores after canopy closure. Those spores can then infect through flowers, stems, or leaves. The pathogen generally needs to infect dead tissue before moving into living tissues, so frequently it enters the plant through dying flower petals and possibly through dying leaves in the lower canopy that are a normal part of the crop growth. Sclerotinia can also cause crown and stem rot, resulting in wilt.

White mold most frequently is introduced to fields in contaminated seed and builds up over a number of years before significant levels of disease occur.

Management

  • Fungicide Applications – After Sclerotinia infects the crop, there is no fungicide that can resurrect the crop. Preventative sprays of Endura (Boscalid) at the bloom period may have efficacy at preventing bloom infection by the fungus, but getting fungicide into the lower canopy to prevent crown and stem rot is difficult if the crop canopy is very dense. Strobilurin fungicides such as Headline and Quadris are not effective at preventing the disease. Note that products including Omega (chickpea only) and Endura (chickpea, pea, lentil) have a 21 day preharvest interval. For effective disease control good coverage in the canopy is critical – it may require high levels of water, nozzles and pressures designed for good coverage deep in the canopy. If you choose to spray, there is no guarantee you will recover yield but you may prevent further infection. Please consult the MSU Extension Foliar Fungicide Table to identify suitable products. Michael Wunsch @ NDSU also has great fungicide efficacy data available for white mold control.
  • Crop rotation – Sclerotinia has a very broad host range of broadleaf crops and weeds. Disease prevention through crop rotation is therefore limited, but rotation to small grain crops (grasses) can reduce the amount of inoculum. However, sclerotia can remain viable in the soil for 8+ years. Pulses included in cover crop mixes may also be a risk factor for disease development.
  • Tillage - Sclerotia can germinate in the top 2” of soil. Tillage buries sclerotia and infected residue and can prevent outbreaks in future years, but additional tillage operations in future years may bring sclerotia back to the soil surface.
  • Weed control - Many broadleaf weeds serve as alternative hosts of Sclerotinia, so good weed control is important to prevent inoculum buildup.
  • Sanitation - it is important to clean equipment after performing any operations in an infected field to prevent spread of the pathogen to additional fields.
  • Prevention - Do not plant seed infected with Sclerotinia, or seed contaminated with sclerotia.
  • Partial host resistance (from M. Wunsch, in 2016) - ‘In a field trial conducted in Carrington in 2012, ‘CDC Viceroy’ (a small green lentil), ‘CDC Impala CL’ (a extra-small red lentil), and ‘CDC Maxim CL’ (a small red lentil) showed excellent performance under high Sclerotinia disease pressure. ‘CDC Impress CL’ (a medium green lentil), and the large green lentils ‘Pennell’ and ‘Riveland’ did not perform as well. ‘CDC Richlea’ exhibited intermediate performance. These results should be treated cautiously; they were obtained from a single trial in a single year, and additional testing is needed for confirmation.’

Where you can find more information

Picture Captions

Figure 1. Brown patches of dead and dying lentil plants due to white mold infection. (Picture credit: Weidong Chen, USDA-ARS Pullman, WA)

Figure 2. Dense, white mats of fungal growth on a white-mold infected lentil stem. (Picture credit: Weidong Chen, USDA-ARS Pullman, WA)

Figure 3. Sclerotia of the white mold pathogen on the soil surface. (Picture credit: Weidong Chen, USDA-ARS Pullman, WA)

Information in this AgAlert were previously published by Mary Burrows in AgAlerts on July 19 and July 21, 2016.

Please don’t hesitate to email (uta.mckelvy@montana.edu) or call (406-994-557) if you have any questions. I’m here to help.

Best,

Uta McKelvy, Extension Field Crop Pathologist