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Peanut (Arachis hypogaea L.) is monocultured in western Nicaragua on loamysand soils, and a pod rot of unknown etiology can greatly reduce crop yield. Pythium myriotylum was frequently isolated from symptomatic pods in fields surveyed at Cosiguina, Leon, and Chinandega regions, although Rhizoctonia and Fusarium were also common. Applications of mefenoxam (0.57 kg a.i./ha), azoxystrobin (0.34 kg a.i./ha), and gypsum (670 kg/ha) at beginning pod and 28 to 35 days later were evaluated in field trials to determine their effects on pod rot and yield. Mefenoxam consistently decreased pod rot incidence and increased yield when disease was severe at Cosiguina. In Leon and Chinandega, azoxystrobin increased yield in fields with little pod rot, apparently by controlling stem rot (Sclerotium rolfsii), but did not decrease pod rot incidence. Application of gypsum had no effect on pod rot incidence or yield, but sometimes increased calcium levels in shells. Pod mycoflora isolations and response to mefenoxam suggest P. myriotylum is the primary cause of peanut pod rot in Nicaragua, especially in Cosiguina, where pod rot incidence was high. Introduction Peanut production in Nicaragua is concentrated in Cosiguina, Chinandega, Leon and, to a lesser extent, Managua. Soils in these regions are typically of volcanic origin (10) and are well adapted for peanut production. Peanuts are grown during the rainy season with very limited irrigated production during the dry season. Cosiguina receives the highest annual rainfall, especially from June to November when peanut is grown. Peanut pod rot is prevalent in Cosiguina and to a lesser extent in Chinandega and Leon. Symptoms included dark pericarp discoloration and moist seed decay (Fig. 1). Estimates of losses due to pod rot in Nicaragua have not been documented, but disease incidence and crop loss can be severe in some years. The etiology of this pod rot is unknown. In other countries, pod rot can be caused by soilborne pathogens such as Pythium spp., Rhizoctonia solani, and Fusarium solani (6,7,8,15). A soil calcium imbalance can also play a significant role in the pod rot disease complex (4,9). Calcium is only available for direct uptake by pods from the soil solution and not from roots or by translocation from foliage (11). This mechanism is the basis for current recommendations to apply highly soluble calcium sources such as gypsum (23.3% calcium) during pod development. The relationship between soil nutrient status and pod rot development in Nicaragua has not been explored, and there were questions of calcium availability in these volcanic soils exposed to frequent torrential rain. Initial studies (1) indicated that calcium deficiency may not be the primary cause of pod rot in Nicaragua, and that plant parasitic nematodes and other soilborne fungi such as R. solani and S. rolfsii were not involved. This study focused on the role of parasitic Pythium spp. and their interaction with calcium availability on pod rot incidence and peanut yield. 15 February 2010 Plant Health Progress Isolation and Identification of Potential Pod Rot Pathogens Ten fields in Cosiguina, four in Chinandega, and three in Leon, were surveyed in 2006 for pod rot fungi. In 2007, seven fields in Cosiguina, four in Chinandega, and three in Leon were also surveyed. The majority of fields were in the Cosiguina region where pod rot is more prevalent. The surveys were conducted at harvest between November and December by collecting freshly dug symptomatic and asymptomatic pods. Pods were considered to be rotted if they had light to dark pericarp discoloration and some degree of seed decay. All fields had been in continuous peanut cultivation for at least 5 years. At each field, approximately 30 pods with attached pegs were collected from arbitrarily selected sites. Samples were placed in plastic bags and transported to the laboratory in coolers for processing. Pod samples were washed three times in running tap water to remove surface contaminants, and 1-cm sections were cut from the shells and pegs of rotted and asymptomatic tissues. These sections and one intact seed per pod were again washed in tap water, rinsed in sterile, distilled water and air-dried on paper towels before placement on PDA medium and incubating at 25 to 28°C for at least 48 h. Oomycetes were sub-cultured on V-8 medium (200 ml V-8 juice, 4.5 g CaCO , 17 g Difco agar, and 800 ml distilled water) for further identification. Potential pathogens were identified, and the isolation frequency of each was recorded for each sample type, region and year. The 2006 season was relatively dry in all regions, especially in Leon and Chinandega where total rainfall in the June to November growing season was 956 mm. Cosiguina had the highest rainfall both years averaging 2,616 mm. Pythium myriotylum and F. solani were isolated more frequently than other potential pathogens with P. myriotylum most commonly isolated from seed with wet decay. This was particularly true in 2007 with higher rainfall totals, and isolation frequencies of P. myriotylum from pegs, shells and seed were high in all regions (Table 1). Distinctive morphological characteristics of P. myriotylum included clusters of large appressoria and swollen sporangia (15) (Fig. 2). Samples with dry seed decay had more F. solani than other fungi (results not shown). Rhizoctonia solani was isolated from seed with wet and dry decay at similar frequencies (results not shown), and was more prevalent in rotted pods from Cosiguina than from the other regions in both years (Table 1). Other fungi isolated from symptomatic pegs, shells, and seed at lower frequencies included Sclerotium rolfsii, Rhizopus spp., Trichoderma spp., and Aspergillus spp. While these species were commonly isolated from asymptomatic samples, P. myriotylum was not. Results of isolation frequencies for R. solani and F. solani from rotted compared with asymptomatic pods were inconclusive (results not shown). Fig. 1. Pythium pod rot with dark pericarp discoloration and moist seed decay in Cosiguina, Nicaragua.

Etiology of Peanut Pod Rot in Nicaragua: II. The Role of Pythium myriotylum as Defined by Applications of Gypsum and Fungicides