Pest of the Month – White Mould in Snap Beans

From ONvegetables in The Grower, December 2013

Beans are susceptible to white mould infections during flowering and early pod-set.  Small, circular, water-soaked lesions develop on the pods of infected flowers, or where fallen petals become caught in the lower canopy or leaf axils.

Infected tissues later develop a dense, cottony, white fungal growth.  Leaves of severely infected plants will eventually turn yellow and fall off.  Sclerotia (hard, black, irregular-shaped fruiting bodies) form in the branches, stems and pods of infected plants (Figure 1).

White mould over-winters mycelium in infected crop residue and as buried sclerotia.  The sclerotia will survive for up to five years in soil and crop residue.

The initial infection period requires moist soils and temperatures between 11 and 20º C (52-68º F.)  A dense crop canopy often produces a cool, moist microclimate, ideal for white mould release. Plant surfaces must remain wet for 24-48 hours for infection to occur. Once established, the disease develops most rapidly between at temperatures between 20 and 25º C (68-77º F.)  Fungal development essentially stops at temperatures over 30º C (86º F.)

If the weather conditions in the crop canopy are conducive to white mould infection, apply a white mould fungicide at 20% bloom followed by a second application 7-days later.

black, pea sized fungal fruiting body
Figure 1. White Mould Sclerotinia
fluffy, which fungal strands on stems, pods and old leaves
Figure 2. White Mould Infected Pods and Stem

2 thoughts on “Pest of the Month – White Mould in Snap Beans”

    1. Lance, Switch and Allegro are all registered. Spray at first bloom (usually around 10-20%) and a second application 7-days later if the conditions are still favourable for white mould development. There is also a biological (microbial) product called Contans which is applied to the soil prior to planting. It suppresses, but does not completely control the disease. Hope this helps!

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