Etiology and management of pod rot and other peanut (Arachis hypogaea L.) diseases in Nicaragua and the United States
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Pod rot is an important disease of peanut (Arachis hypogaea L.) in the pacific coast region of Cosiguina in Nicaragua, but the etiology was unknown. Surveys in 2006 and 2007 showed that Pythium myriotylum was the most commonly isolated species from rotted peanut pods in various locations. Field experiments conducted from 2005 to 2007 showed that mefanoxam was the most effective treatment with 57% less pod rot and 13% yield increase compared to control plots. Supplemental calcium had no effect on pod rot, and controlling lesion nematodes increased pod yield, but also did not reduce pod rot. These results suggest that P. myriotylum is the most important pod rot factor in Nicaragua. Stem rot (caused by Sclerotium rolfsii) is another important disease of peanut in Nicaragua. Peanut growers there plant very high seeding rates similar to, or often higher than, those recommended in Georgia (19.7 seed m-1) to reduce risk of tomato spotted wilt virus (TSWV). However, TSWV has not been reported in Nicaragua, and high density plant stands can exacerbate stem rot. Field experiments in 2005 and 2006 demonstrated increases in stem rot with denser plant stands in fields with significant disease incidence. Gross income adjusted for seed cost and peanut yield increased with increasing plant stands up to 8 – 11 plants m-1 and decreased at higher plant densities. In locations with low S. rolfsii prevalence, maximum yield and gross income adjusted for seed cost were attained at 12 plants m-1. Stem rot is an important disease for peanut growers in Nicaragua and the United States, and growers usually spray fungicides to control it. Fungicides applied in the evening (8-9 pm, with folded and dry leaves) or morning (3-5 am, with folded and wet leaves) were more effective than those applied during the day (10 am-12pm, with unfolded and dry leaves). Morning sprays gave the greatest increase in disease control and pod yields compared to day sprays, and further studies documented increased spray deposition in the lower canopy where stem rot infections occur, as well as longer residual activity of fungicides on the shaded versus the sun-exposed leaves.