Etiology of Peanut Pod Rot in Nicaragua: I. The Effect of Pod Size, Calcium, Fungicide, and Nematicide

Pod rot is one of the most important peanut (Arachis hypogaea L.) diseases in the pacific coast region of Cosiguina in Nicaragua. The region receives excessive rainfall during the peanut growing season and fields with high pod rot incidence often have moderate to high infestations of stem rot (Sclerotium rolfsii) and lesion nematode (Pratylenchus spp.). It was hypothesized that soilborne fungi, nematode damage and/or soil calcium deficiency could be involved in peanut pod rot. Flutolanil (1.2 kg a.i./ha, two applications), aldicarb (3.4 kg a.i./ha, one application), and calcium (670 kg/ha, two applications of gypsum) were evaluated on small (Georgia Green) and large (C-99R) seeded, runner cultivars in field experiments from 2005 to 2007 to help determine disease etiology. Experiments were split-split-plot or split-plot designs and treatments were replicated five times. There were no differences in pod rot between the two cultivars. Flutolanil did not decrease pod rot and only increased yield when associated with aldicarb nematicide. Gypsum application did not decrease pod rot, or increase yield or pod calcium content. Aldicarb had no effect on pod rot, but significantly increased yield by 22%, apparently by suppression of pod and/or root damage by lesion nematode. Neither calcium deficiency nor lesion nematode damage increased pod rot. Introduction Pod rot is one of the most important diseases affecting peanut production in Nicaragua. The disease is more prevalent in the pacific coast region of Cosiguina, as well as in Chinandega and Leon to a lesser extent. Peanut pod rot symptoms include brown to dark discoloration of the pericarp and pegs that usually begins at the apical pod tip and extends to the basal pod segment and peg. In addition to rotted pods, pegs often disintegrate and pods are lost in soil, further contributing to a yield reduction (7). Preliminary observations in locations with high pod rot incidence in Nicaragua showed that lesion nematode populations were often high. Laboratory assays of pods with symptoms of lesion nematode damage (Fig. 1) indicated densities of 1456 to 2462 Pratylenchus spp. per 5 g of shells of Georgia Green. Lesion nematode injury can best be identified by the presence of small tan spots with dark centers on pods. The lesion nematode is common in the warmer climates of the world similar to that of Nicaragua, and can be found in large numbers inside the shells of mature peanut pods and pegs (15), where they colonize dark-colored, necrotic lesions (10). In a heavily infested field, Good et al. (10) found that peanut shells of a runner cultivar contained from six to eight times as many P. brachyurus nematodes as did the roots. The nematode feeds within the parenchymatous tissues, weakening the pegs and pods, and is recognized as an economically important pathogen of peanut in most production regions of the world (17). Although other nematode species have been implicated in the pod rot complex of peanut (8), no literature is available for pod rot caused by interactions between Pratylenchus spp. and 15 February 2010 Plant Health Progress fungi. Nevertheless, Good et al. (10) found that in fields heavily infested by P. brachyurus, 19% of pods were left in the ground at digging because of weakened or rotted pegs associated with heavy infection by lesion nematode. Conversely, only 6% of the pods were left in soil in plots treated with nematicide. Since many fields with pod rot in Nicaragua also had high populations of lesion nematodes, it was important to determine the relationship between lesion nematode damage, peanut pod rot, and yield. Stem rot (Sclerotium rolfsii) is another important soilborne peanut disease in fields with pod rot prevalence in Nicaragua. Sclerotium rolfsii infects lower stems, roots, pegs, and pods during peanut pod formation and maturation, or earlier, and may cause pod rot (16) (Fig. 2). The association between S. rolfsii and other fungi in peanut pod rot has been reported as well (12), but not between S. rolfsii and Pratylenchus spp. nematodes. Flutolanil, a benzanilide fungicide, is registered for use on peanut and effective against S. rolfsii and Rhizoctonia solani (4), another fungal species isolated from pod rot samples from peanut fields in Nicaragua. Other isolated fungal species and their significance to peanut pod rot in these fields are discussed in a second manuscript of this series (2). Another factor that has been shown to play a significant role in the peanut pod rot complex is low level of available calcium in soil (5), especially in largeseeded cultivars (9,11,14). Sumner et al. (18) indicated that pod surface area and surface to volume ratio were important in determining the quantity of calcium in seed and explains why large seeded varieties are more sensitive to calcium deficiency. Georgia Green, a small-seeded runner type, is the standard peanut cultivar in Nicaragua. However, other peanut cultivars with different pod and Fig. 1. Lesion nematode damage on peanut pods caused by Pratylenchus spp. in Cosiguina, Nicaragua. Fig. 2. Peanut pod rot caused by Sclerotium rolfsii in Cosiguina, Nicaragua. Note the uniform discoloration of infected pods, sclerotial initials of S. rolfsii, and the premature seed sprouting, which are typical of pod rot associated with S. rolfsii. 15 February 2010 Plant Health Progress seed sizes are being introduced to the country. Nearly all of the new cultivars have larger pods than Georgia Green (809 to 875 seeds/kg), with one of the largest being C-99R (605 to 678 seeds/kg). Due to its larger seed size, C-99R should be more susceptible than Georgia Green to pod rot induced by calcium deficiency. The relative susceptibility of these new cultivars to pod rot and comparing a large versus small-seeded cultivar would be an indicator of the importance of calcium nutrition to pod rot development in Nicaragua. The objectives of the study were to understand the etiology of pod rot in Nicaragua, and to determine the effect of cultivar selection, fungicide, nematicide, and gypsum applications on pod rot and peanut pod yield. Cultivar Selection and Fungicide, Nematicide, and Gypsum Treatments Field experiments were conducted in seven locations in Cosiguina and one location in Leon regions in Nicaragua from 2005 to 2007 to evaluate the effect of cultivars (small-seeded Georgia Green and large-seeded C-99R), flutolanil, aldicarb, and gypsum applications on pod rot control and peanut pod yield. These inputs were chosen to selectively control factors known to cause pod rot, and identify those with the greatest impact in Nicaragua. Sclerotium rolfsii and R. solani were suspected to cause pod rot of peanut in Nicaragua, and flutolanil is one of the most effective fungicides used for disease control (4). Aldicarb is labeled for use on peanuts to control nematodes. It also has activity on thrips, but thrips damage has not been observed on peanuts in Nicaragua. Applications of gypsum were evaluated as a source of calcium, and were repeated to compensate for leaching by heavy rainfall. Fields were selected based on past occurrences of pod rot and high populations of lesion nematodes. The fields were prepared by disk plowing 20 to 25 cm deep and disk harrowing three times before planting. The experimental design was either a split-split-plot or split-plot with treatments replicated five times. Each plot consisted of double beds (two twin rows per bed), the left bed for pod yield and the right bed for destructive sampling for nematode and pod rot assessments. Plots were 10 m long and separated by a 3m fallow alley. Georgia Green and C-99R were planted with a twin-row Cole planter at 23 to 26 seeds/m. Row spacing was 109.2 cm between the outside rows and 73.7 cm between the inside rows. In split-split-plot experiments, the whole-plot factor was fungicide, the subplot factor was nematicide, and the sub-sub-plot factor was gypsum. In the split-plot experiments, the whole-plot factor was cultivar and sub-plot factor was nematicide. Fungicide treatments were: flutolanil (1.2 kg a.i./ha); and nontreated control. Nematicide treatments were: aldicarb (3.4 kg a.i./ha); and nontreated control. Gypsum treatments were: at beginning pod (R3, 50 to 60 DAP) and beginning seed (R5, 80 to 90 DAP), as defined by Boote (3), with application of 670 kg/ha each time; and nontreated control. The gypsum was applied by hand in a 50-cm band centered over the row without incorporation. The field sites had a medium level of extractable calcium with ammonium acetate extraction method (6) prior to planting (7.8, 7.6, and 7.4 meq Ca/100g soil in 2005, 2006, and 2007, respectively), but it was hypothesized that soil calcium deficiency might occur during pod formation and maturation by leaching of calcium during excessive rainfall or the calcium being in an unavailable form. Flutolanil was applied at the same times as gypsum with a CO -beltpack sprayer delivering 189 liter/ha at 31 psi by four 110 04 tips with 50 mesh screens per bed. Aldicarb was applied after planting, but before seedling emergence, with a Gandy applicator in a 30-cm band centered over the row and lightly incorporated with a rake. Standard production practices were applied uniformly to the field, including regular sprays with chlorothalonil (1.26 kg a.i./ha) for control of leaf spots and peanut rust. Plots were not fertilized or irrigated during the growing seasons. Pod rot incidence was determined by counting the number of intact and rotted pods collected from plants in 1-m sections of each bed. Rotted pods left 2 15 February 2010 Plant Health Progress in soil were manually excavated and included in counts. Since some pods would have been destroyed before harvest at full maturity, this evaluation was conducted approximately 100 DAP. Disease incidence was obtained by dividing the number of rotted pods by total number of pods in a section and multiplying by 100. Three 1-m sections were sampled in ea