What is in this article?:
• Fungicides work primarily as a preventive.
• Once you have a disease, it’s a lot harder to manage than if you head it off with a proper application of fungicide.
• Growers should seek to optimize fungicide use in order to hold the line on costs.
ALAN ORTLOFF, from left, president of the Clint Williams Company, Madill, Okla.; Marshall Lamb, director of the USDA/ARS National Peanut Research Laboratory, Dawson, Ga.; and Lee Talbot, Brooks Peanut Company, Samson, Ala., were among those attending the annual meeting of the Mississippi Peanut Growers Association.
Controlling leaf spots
Early leaf spot spread can be controlled with a regular fungicide application program, Henn says. “This is one of the diseases that has forced adoption of calendar spray programs in Georgia and Alabama. It is manageable disease — but it is not one to be ignored.
“Scouting of fields and appropriate fungicide sprays will prevent loss. Chlorothalonil fungicides (Bravo, Echo and others) protect the leaf surface against new infections, whereas some of the penetrant-type fungicides that enter plant parts near the droplet have some ‘curative’ activity — they can kill the fungus after it has infected plants, but before it spreads.
“Fungicides include but are not limited to Headline, Provost, and Tebuzole. They have little activity against infections already present, and all start losing activity 7-10 days after application. You can improve how well all fungicides work by increasing spray volume; 20 gallons to 30 gallons per acre will be more effective than 15 gallons per acre.
“If a field already has significant defoliation, 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.”
Funky leaf spot
There is no identified cause of funky leaf spot, also called Florida leaf spot and irregular leaf spot, Henn says. “Extensive laboratory examination of FLS symptoms has yet to find a cause, bacterial or fungal. Leaf symptoms look very much like leaf scorch.
“FLS leaf symptoms are common on lower plant leaves and are most common during the earlier part of the season, before the plant really gets into reproductive growth. There may be a greater incidence in fields with reduced tillage or residue amounts. I’ve noted that FLS is more common in areas where moisture stays on the leaves longer, as on the eastern side of the field near trees, or where an upper leaf touches a lower leaf, and where a plants are already stressed.”
No clear association between FLS and yield has been established, Henn says. “Defoliation can be high in some older cultivars, but I have yet to see a serious defoliation problem in Georgia 06 G.
“A number of people have looked at applying fungicides for control of FLS, with no effect. Save your money and fungicides — don’t try to control this leaf spot; it will disappear on its own.”
Leaf scorch/pepper spot
A fungus, Leptosphaerulina crassiasca, can cause two different disease symptoms on peanut leaflets — leaf scorch and pepper spot, Henn notes. “Both symptoms produce only minor plant problems (slight defoliation), and by themselves are not worth spraying. Sprays that are targeted to other problems will almost inevitably control this fungus.”
Leaf Scorch is the most common symptom produced by the fungus in Mississippi, he says.
Pepper spot, also caused by the fungus, is identified by small (about 1 mm) black spots on the upper side of the leaf that grow slowly.