Sclerotinia blight of peanut
updated June 17, 2011
Sclerotinia blight (caused by Sclerotinia minor) is found in about 25% of North Carolina peanut fields, and is particularly prevalent in Northeastern North Carolina. It has not been found in new production areas in the Central Coastal Plain, but growers should assume that the disease could become established in any peanut field in North Carolina.
Because this disease starts by killing individual limbs, careful scouting is required to see symptoms when they first appear. Tips of infected limbs may remain green and look healthy for several days before wilting is evident. Vines must be pulled back to reveal bleached stems and the white cottony growth of the Sclerotinia fungus (Figure 1). Infections can be found on stems, pegs, and leaves. The fungus is most visible on humid mornings and after a rain. Eventually, stems become shredded and die. Small black irregularly shaped sclerotia that resemble mouse or insect droppings may be present on and in infected stems and pods (Figure 2).
Conditions that favor the disease
Sclerotinia blight is favored by cool (65 - 75 degrees F), moist conditions. As soon as peanut rows begin to touch, conditions within the canopy become cool and humid, and thus favorable for infection. Even a short cool spell can be sufficient for infection to occur once the canopy begins to close. If weather turns hot and dry, the disease may not advance, but at least some the infections remain active. When cool wet weather returns, disease can increase rapidly from previous and new infections.
Injury from tractor traffic can make plants more susceptible to infection. Frequent application of chlorothalonil (Bravo, various brands) can make Sclerotinia blight difficult to control and should be avoided in fields with a history of Sclerotinia blight. Sclerotinia blight becomes more severe as soil pH increases from 6.0 to 6.5.
To prevent build-up of damaging levels of Sclerotinia
blight, rotate as long as possible with cotton or corn. Canola and many
cool-season vegetables should not be used in rotations. In addition, many
common winter annual weeds are hosts. They support reproduction of the fungus
during winter fallow, potentially reducing the benefits of rotation. Planting a
small grain cover crop may reduce populations of the weeds that host the
Sclerotinia blight pathogen.
Avoid susceptible cultivars in fields with a history of
disease. The cultivars Bailey and Sugg have higher resistance to Sclerotinia
blight than older, moderately resistant cultivars, but still require fungicide
applications under heavy disease pressure.
Minimize trips across the field to reduce vine injury and
minimize use of chlorothalonil for leaf spot control. Carefully weigh all plant
health factors when applying lime to fields where Sclerotinia blight is a
problem. Sclerotinia blight can be seed borne, so always buy fungicide-treated seed
from from a reputable supplier.
The fungicides fluazinam (Omega) and boscalid (Endura) are effective against Sclerotinia blight when applied as preventatives. Timing of the first spray is critical. Fields with a history of serious problems should be scouted carefully beginning when vines are within 6 inches of touching, or around July 4. Treat when Sclerotinia blight is first observed (on demand) or 60 to 70 days after planting (calendar program) or according to a Sclerotinia blight advisory. A weather-based Sclerotinia blight advisory can be used to time applications and prevent unnecessary fungicide applications. If the disease continues to spread, one or two more applications may be made at 3- to 4-week intervals, or according to the advisory.
Consult your county agent for more information about Sclerotinia
blight advisories or to receive Sclerotinia advisories by e-mail. See the North Carolina Agricultural Chemicals Manual for fungicide rates and application information.