Early leaf spot found in the greater Aberdeen area

Description and symptoms.  Yesterday I looked at some defoliating fields in the greater Aberdeen vicinity. The vines looked like someone had used a set of hedge shears to remove the foliage from between the rows and thin it in the center of the row (Figure 1). The foliage had been fully covering the center of the rows as recently as 7- 10 days ago.

Survey of the peanut canopy showed that many leaflets had leafspots (see Figure 1, left foreground and left row). By itself, this is not unusual since we have been seeing many dark brown spots or the dark brown spots surrounded with a thick, diffuse, yellow ring caused by leaf scorch, and anthracnose combinations. While some leaf spots had no yellow ring around them, a thin yellow ring was present on most (Figure 2 and 3). Distinguishing these leaf spots, was a growth of gray tufts inside the lesion (Figure 2 and Figure 3) when it was examined with a 12x hand lens. The growth is a distinct gray, or in bright light, silvery color. These are fruiting bodies of the fungus. Laboratory examination confirmed the field diagnosis of Early Leaf Spot (Cercospora arachidicola). Also present were lots of spots caused by leaf scorch, a fungus we have seen since early season.  Leaf spots caused by leaf scorch and those by early leaf spot were not easily separable, unless fruiting bodies were seen.

I visited about four fields that were within about 1 mile of each other (as the crow flies). Some fields had only a few leaf spots (more recently infected), whereas one had about 20-25% of the leaves with leaf spot.  This probably means that time of infection varied among the fields.  The thinnest canopied field had been heavily grazed by deer, and between the browsing and defoliation caused by the leaf spot, had a reduced canopy (Figure 4).  The grower thinks the infection may have started here.

Understanding the pathogen, approximating infection date and location of infections. Early leaf spot infects the plant best during times of high humidity, free moisture, and cooler temperatures (low 70°’s F). For at least some isolates, optimum growth is about 77° F and maximum temperature of growth is about 88° F. Latent periods (the time between infection and symptom expression) are between 13-39 days, depending upon temperatures. Since the grower first noted the leaf spot in the heavily browsed field almost exactly 2 weeks ago, The latest an infection could have occurred (probably during a rain event), was a minimum of 4-6 weeks ago (about July 24 through August 7), and given our temperatures, probably occurred during the unseasonably cool period we experienced in July. The rain and temperatures surrounding Isaac may have favored spread and development of this leaf spot. This possibility is one reason I advocated good fungicide spray coverage prior to the storm. But remember, that this find in the greater Aberdeen area does not mean other peanut growing areas in the state are infected. Scouting of your fields and appropriate fungicide sprays will prevent loss. If you run into a leaf spot or lesion you are unsure of, I will be glad to look at some leaves.

Management. Early leaf spot progression can be stopped by using a regular spray program. This is one of the diseases that has forced the calendar spray programs common in Georgia and Alabama, so it is a disease that is manageable, but is not to be ignored. Scouting of your fields and appropriate fungicide sprays will prevent loss. Remember that chlorothalonil fungicides (sold as Bravo, Echo and others) protect the surface of the leaf against new infections, whereas some of the penetrant type fungicides (enter plant parts near the droplet) have some “curative” activity. That is they can kill the the fungus after it has infected, but before it expresses. Such fungicides include (but are not limited to) Headline, Provost, Tebuzole. They have little activity against infections already expressed (seen), and all start losing activity 7-10 days after application. You can improve how well all fungicides work by increasing spray volume. Twenty to thirty gallons per acre are more effective than 15 gallons per acre. If a field is already significantly defoliating, early leaf spot is common on the remaining leaves, and the crop is approaching maturity (as judged by color and number of the blasted nuts), you may chose to dig affected fields first.