Effects of intercropping corn and peanut on peanut leaf spot management and the spatial and temporal epidemiology of Cercospora arachidicola.

Abstract

Peanut-corn intercropping was evaluated for its potential to reduce early leaf spot (ELS) of peanut, caused by Cercospora arachidicola. Peanut (Arachis hypogaea) was intercropped in field plots with corn (Zea mays), and the effects of intercropping on dispersal of C. arachidicola and spatial dynamics of disease were examined over time. In 2000, the experiment consisted of five replicate blocks of square plots 16 rows wide and 14.6 m long. Treatments included unsprayed peanut (p) monoculture (Monocrop), sprayed peanut monoculture (Monocrop Sprayed), alternating rows of peanut and corn (c) (Intimate Intercrop), and four-row strip intercrops (2c, 4p, 4c, 4p, 2c) (Strip Old). In 2001, a second strip intercrop treatment was added (4c, 4p, 4c, 4p) (Strip New) and plots were 15.4 m long. Corn and peanut (VA 98R) were planted on May 9, 2000 at the Horticultural Crops Research Station near Castle Hayne, NC, and May 10, 2001 at the Umstead Farm Unit near Butner, NC. Both locations are outside of normal peanut production areas but are suitable for peanut culture. In late July (2000) and in mid August (2001), focal epidemics were initiated by placing infected peanut stems centrally in each plot. Leaf spot incidence and defoliation were determined weekly in a stratified sampling routine that allowed estimation of disease gradients in four directions. Airborne conidia were trapped with a Rotorod spore sampler in three blocks of all treatments except the Monocrop Sprayed. Peanuts were dug at maturity, and yield data were taken. Corn yield was estimated in 2001.

Early leaf spot symptoms were first observed near the inoculation site 22 days after inoculation in 2000 and 23 days after inoculation in 2001. In 2000, Intimate Intercrop and Monocrop reached the highest mean level (averaged across distance and direction) of disease incidence at around 41% by 63 days after inoculation. Disease incidence AUDPCs for Intimate Intercrop and Monocrop were significantly greater than the AUDPC for Strip Old intercrop, which was significantly greater that the AUDPC for Monocrop Sprayed. Natural populations of Cercosporidium personatum caused a nonpoint source late leaf spot (LLS) epidemic on the peanuts. In 2001, Monocrop again reached the highest level of disease incidence at 15%, 62 days after inoculation. Disease incidence AUDPCs for Monocrop were significantly higher than the other four treatments. Intimate Intercrop had an intermediate AUDPC, which was significantly greater than the two Strip treatments and the Monocrop Sprayed. The two Strip intercrop treatments also had intermediate AUDPCs. The Strip New treatment had significantly greater AUDPC values than the spray treatment. The Strip Old treatment had a statistically similar AUDPC to the sprayed treatment. Another non-point source late leaf spot epidemic occurred, but was much less severe than in 2000. Peanut yield was not significantly affected by either disease or intercropping during 2000. In 2001, peanut yield patterns were similar to those in 2000, but the Intimate Intercrop was significantly depressed. No benefit or reduction was observed in corn yield due to intercropping.

Apparent infection rates (r) were calculated as the slopes of the linearized logistic model applied to the disease progress curves. A repeated measures analysis revealed a treatment x distance interaction (p<0.05) during the last four sampling days of each ELS epidemic. This likely was due to the Monocrop Sprayed treatment, which maintained low disease levels at all distances from the inoculum. Disease gradients were plotted for each treatment/day combination, and log-linear, log-log, logit-linear, and logit-log models were applied to gradients. In both years, best fits were obtained when log distance was used. The logit-log model was selected in 2001 due to higher disease levels and the increased likelihood of multiple infection. In 2001, the log-log model was used because multiple infections probably did not limit disease spread. Linearized daily slopes within each treatment were analyzed with ANOVA and those that were significantly different from the majority of others in that treatment were discarded. Averaged individual plot slopes from the remaining days were used to estimate gradient parameters (b) for that treatment. Velocity (V) then was calculated according to the formula Vs=rs/b using the averaged values of b obtained previously and arbitrarily chosen distances (s). Velocity increased with increasing distance from the inoculum source. At s=1, velocities during both years ranged from 0.40 to 0.96 over all treatments with the exception of Strip New, where V=0.222. In 2000, Strip Old had the highest velocity at all distances, followed by Monocrop. Intimate Intercrop had the lowest velocity at all distances. In 2001, Strip New had the highest velocity at all distances from the source of inoculum. Velocities were comparable to those previously reported for LLS of peanut.