Early leaf spot of peanut
Updated June 17, 2011
Early leaf spot of peanut is caused by the fungus Cercospora arachidicola. Early and late leaf spots are the most common and important peanut diseases. In North Carolina, leaf spot control is necessary to prevent defoliation and yield loss on all peanut cultivars.
Early leaf spot typically causes brown lesions (spots) on leaves. The spots usually are surrounded by a yellow halo, but sometimes halos are not present. The brown lesion color is most distinct on the lower leaf surface (Figure 1). The fungus produces tufts of silvery, hair-like spores on lesions during humid weather. These spores can be seen on the upper lesion surface with the help of a good magnifying glass (Figure 2).
Although early leaf spot can be found as early as 30 days after planting, it usually is not observed in well-rotated fields until mid-July or later. Spots are first seen on lower leaves. As the disease progresses, the number of spots increases. Plants begin to defoliate, starting with the lower leaves and progressing to the upper canopy. In severe epidemics, plants are completely defoliated (Figure 3). The bare stems may also have lesions.
Conditions that favor disease
Leaf spots develop most rapidly when prolonged periods (8 or more hours a day) of very high relative humidity correspond with temperatures in the 70's (degrees F). These conditions are very typical at night or during rainy days in North Carolina's summer months.
Peanut cultivars vary in susceptibility to both early and late leaf spot; the virginia-type cultivar Bailey and some runner-type cultivars have moderate leaf spot resistance. Fungicide applications are still necessary to control leaf spots on these cultivars, but fewer spays may be possible.
- on a set 14-day calendar schedule, OR
- according to a weather-based leaf spot advisory.
In well-rotated fields, the first fungicide spray should be applied to susceptible cultivars at the very early pod stage (R3), which usually occurs in the first week of July. Sprays may be delayed for 2 weeks on Bailey. After the first spray, apply fungicides every 14 days and continue applications until early September (about 3 weeks before digging), for a total of 5 sprays in most years.
Alternatively, peanuts can be sprayed according to the leaf
spot advisory. The advisory is a safe way to minimize fungicide applications by
spraying only when weather conditions favor disease. Eliminating unnecessary
fungicide sprays during dry spells helps to prevent spider mite flare-ups and
can reduce tractor damage to vines, making them less prone to Sclerotinia
blight, Rhizoctonia limb rot, and Botrytis blight.Contact your County Agent for more information about leaf spot advisories.
Leaf Spot Fungicide Resistance Management
Continued use of certain leaf spot fungicides may cause resistant strains of leaf spot fungi to build up to damaging levels. Many common peanut fungicides belong to group 3 or group 11 and are prone to select for resistant strains. Strains resistant to a particular fungicide usually will be resistant to all fungicides within the same chemical group or mode of action. To prevent the development of fungicide resistance, it is important to mix or alternate sprays with fungicides from different groups. The group number is prominently displayed on the fungicide label. To maintain fungicide efficacy:
- Mix or alternate sprays with fungicides from different group numbers.
- Do not use site-specific fungicides at less than the recommended rates.
- Do not exceed the total number of sprays allowed for a particular fungicide or group number.
- Use a multi-site fungicide, such as chlorothalonil, for the first and last spray of the season.
- Maintain a good foliar disease control program throughout the growing season.
- NEVER rely on "rescue" treatments to clean up foliar disease problems.
See the North Carolina Agricultural Chemicals Manual and fungicide labels for additional information about fungicide rates and resistance management recommendations.