By Travis Faske, Extension Plant Pathologist
Based on the current estimates there has been about 29,000 acres of peanut planted in Arkansas, which is similar to that of the 2017 cropping season. The peanut crop ranges in growth stage ranges from blooming to early pod development with the majority of the crop at pegging. Although the lack of moisture in some areas is the immediate limiting factor, growers will soon be making decisions on their first fungicide application. With peanut, field history and canopy coverage are important factors when considering a fungicide to manage southern blight, the most common peanut disease in Arkansas.
Southern blight can be identified in the field by signs (seeing the fungus) and symptoms (plant response to disease) of the disease. Scout fields with a history of the disease for white hyphae on the soil surface near the peanut limbs. With that said, a word of caution, Phanerochaete is a southern blight fungus doppelganger (Fig. 1), so look close to make sure its southern blight fungus, Sclerotium rolfsii. Phanerochaeteis a wood rotting fungus that poses no threat to peanut. It is characterized as having “tooth like” projections and as it matures the fungus turn from white to yellow in color. Typically, this disease imposter is the first white fungus observed in June in peanut fields. The southern blight fungus is usually first observed in July in peanut as white hyphae followed by wilted or blighted (rapid death of leaves) leaves (also called flagging) (Fig. 2). As the disease progresses other signs appear on the limbs and soil surface as white sclerotia that turn tan then brown as they mature. These mustard seed size sclerotia act as the overwintering structure for the disease.
In fields where the canopy has yet to meet the threat of southern blight is lower than where the limbs have met across the furrow. In fields where the canopy has met a micro climate is created near the soil surface that promotes fungal development. Though fungicide treatments often begins in July to control southern blight, scout fields for crop maturity and disease signs and symptoms to confirm the need for starting a fungicide program. Applying fungicides to control southern blight before a rain or overhead irrigation is a good idea to get the product near the soil surface. Some apply fungicide at night when peanut leaves are closed when overhead irrigation is not an option. Relying on a calendar program alone could open a window at the start of the season for late leaf spot in September, see last paragraph. See MP154 for fungicides used to control southern blight in Arkansas.
Prior to July, the most common disease observed in the state is Aspergillus crown rot. This disease is caused by Aspergillus niger, which is a very common fungus found in all fields. This is a relatively minor disease of peanut and requires no treatment for control.
It is very common to find spots on peanut leaves but most are NOT caused by the leaf spot of peanut disease (Fig. 4). Peanut is a new crop for Arkansas growers and overall, the disease pressure is low. During the past three years when leaf spots diseases have been reported in the state, it has been in September. This does not mean that leaf spots cannot occur earlier, but that the weather conditions in September were more favorable for leaf spots, especially late leaf spot of peanut, the most common leaf spot of peanut in Arkansas (Fig. 4). With late leaf spot, spores can be seen on the bottom (and to a lesser extent on the top as little bumps in the spot) of the lesion (i.e. spot) for late leaf spot like those for frogeye leaf spot of soybean and on the upper leaf surface (tufts of spores) for early leaf spot of peanut. So, don’t spray just because you see spots, make sure it is leaf spot disease variety. Also, as a general rule the “TUFRunner” peanut cultivars are more susceptible to leaf spot diseases than those from “Georgia”. So, pay close attention to the susceptible cultivars as we progress through the cropping season. See MP154 for fungicides used to control leaf spot in peanut.