There are anecdotal claims that some fungicides cause physiological peg strength enhancement beyond mere suppression of the diseases, which can reduce peanut peg strength. We tested eleven fungicide treatment programs for effects on the peg strength of harvestable pods (NC-V11 cultivar). Peg strength comparisons also were made for pods of different maturity categories based on mesocarp color. Fungicide programs were highly effective in protecting yield (1,690–2,220 kg/ha increase over the nontreated check) and preventing pod loss from late leaf spot and southern stem rot, however none of the fungicide treatments had any measurable effect on the peg strength of healthy (disease asymptomatic) pods. A tebuconazole program failed to prevent defoliation from late leaf spot.
Pods symptomatic for southern stem rot had peg strengths only about 45% that of healthy pods. In contrast, pods symptomatic for tomato spotted wilt had significantly stronger pegs than those of healthy pods. Fully mature (black mesocarp) pods had peg strengths (6.70 ± 0.10 newtons) as great or greater than that of less mature brown (6.29 ± 0.12), orange (6.17 ± 0.14), or yellow (5.54 ± 0.35) mesocarp pod categories. Over-mature pods (characterized by a coal black mesocarp, tan-brown seed coat, and a slight anthocyanin pigmentation on the pod exterior) had a mean peg strength (2.22 ± 0.08 newtons) only about 32% that of fully mature pods. Pegs of over-mature and diseased pods generally broke proximal to the point of pod attachment, while pegs of healthy pods broke at the point of pod attachment and had pod exocarp remnants attached to the pegs.
The data indicate that growers should make fungicide treatment decisions based on disease prevention efficacy rather than on the assumption of any additional physiological peg strengthening benefits. In the absence of disease, we found no decline in peg strength associated with advancing pod maturity until pods could be visually identified as over-mature. These results may prove useful in refinement of harvest timing guidelines based on the distribution of pod maturity as defined by mesocarp color categories. The results could also be useful in helping growers interpret the cause of pod loss.
Foliar-applied of peanut,
Previous studies of factors influencing peg strength have included evaluations of cultivar differences (
Tests were conducted on NC-V11 cultivar during the 2003 and 2004 growing seasons at the Edisto Research and Education Center (Barnwell County, SC). The soil type was a Varina sandy loam (clayey, kaolinitic, thermic, Plinthic Paleudults). The experimental design was a randomized complete block with five replicates of each fungicide treatment. The experimental unit was a plot eight rows wide (0.96-m row spacing) by 12 m long. The middle four rows of each plot were not subjected to traffic after planting in order to reduce experimental error on rows evaluated for disease incidence and harvested for yield and grade. Peanuts were produced using standard practices for virginia type cultivars in conventional tillage (
In 2003 six fungicide applications were made at about 15-day intervals for each of five tested programs except the nontreated check. In 2004 five total fungicide applications were made at about 15-day intervals for each of eight programs except the nontreated check. Complete descriptions of the tested fungicide programs with active ingredient rates and dates of application are listed in
Peg tensile strength was measured with a Shimpo DFS-50 digital force gauge (Shimpo Instruments, Nidec-Shimpo America Inc., Itasco IL). The gauge was mounted on a board and each tested pod was placed in an alligator clip attached to the gauge. The clip allowed pods to rotate slightly as initial tension was placed on the pegs. Pods were placed in the clip such that when pegs were pulled along a reference line on the board, the peg would be normal to the surface of the pod at the point of peg attachment. The peg was then slowly pulled by hand until it either broke along the length of the peg, or more typically detached near the pod. The gauge recorded the peak force required for the peg to fail. After the pod was removed from the clip, the upper surface or “saddle” area of the pod was scraped with a knife to categorize pod mesocarp color as dark yellow, orange, brown, or black. This sequence of mesocarp colors is indicative of advancing pod maturity from the yellow through black stages (
Pods were collected for testing by using a pitchfork to uproot plants as gently as possible. About 0.5 m of row was lifted from each of rows two and seven in each 8-row plot after the middle four rows had been harvested for yield. Pods were cut from plants with scissors, leaving as much peg as possible attached to the pod. In 2003, 20 healthy (asymptomatic for disease), full-sized pods were collected from each plot (100 per fungicide treatment). In 2004, 25 such healthy pods were collected from each plot (125 per treatment). Five over-mature pods were also collected from each plot (25 per treatment) in both years. Over-mature pods were identified by the presence of a slight anthocyanin pigmentation on the pod exterior. These pods were subsequently found to have a coal black mesocarp and tan-brown coloration on the seed coat. Over-mature pods also typically had some visible deterioration of the peg. In 2003, pods were collected with southern stem rot symptoms, as defined by the presence of characteristic mycelia or sclerotia. Invariably these diseased pods also had some visible deterioration of the peg. An attempt was made to collect five southern stem rot symptomatic pods from each plot (125 total), but only 85 were collected because fewer could be found in some of the more efficacious soilborne disease treatments. In 2003 we also collected 120 pods symptomatic for tomato spotted wilt, approximately five from each plot. Pods symptomatic for tomato spotted wilt were identified based on a typical orange color and corky texture of the pod exterior, as well as finding the characteristic foliar ring spot symptoms on the plant (
In 2003 peg strength measurements were taken from 10–15 Oct (147–152 DAP) and in 2004 from 12–18 Oct (148–154 DAP). However, all treatments within a replicate were collected and tested on the same day.
Southern stem rot incidence was rated within two hr of digging by scanning two rows per plot and counting the total row length symptomatic for this disease. Plots were examined for the presence of early leaf spot and late leaf spot within one wk prior to harvest. Two observers scanned the middle four rows of each plot and estimated percent defoliation. Although both leaf spot diseases were present, late leaf spot was predominant and caused the observed defoliation. Stunting from tomato spotted wilt virus affected less than an estimated 5% of plants, and therefore no individual plot ratings were taken for this disease.
The middle four rows of each plot were inverted with a KMC peanut digger (Kelly Manufacturing Company, Tifton, GA) on 8 October (145 DAP in 2003, 144 DAP in 2004). These rows were subsequently harvested with a two-row Hobbs 525 combine (Hobbs Manufacturing Company, Albany, GA) modified with a bagging attachment. Samples were weighed in the field and a subsample (~1,500 gm) was removed for grading. Grade samples were dried at approximately 32 C and then stored at room temperature until graded in accordance with USDA standards (
The data were subjected to analysis of variance (PROC GLM,
None of the fungicide treatments in either year had a significant effect on peg strength when healthy pods were compared (
Results for pod maturity were pooled over years because there was no significant interaction between year and pod maturity (
Pods symptomatic for southern stem rot had significantly weaker pegs, being only about 45% the strength of healthy pods (
On 98% of healthy mature pods, the peg broke at the point of attachment to the pod; whereas on over-mature pods, 60% of the pegs broke proximal to the point of attachment, that is, along the length of the peg rather than at the point of pod attachment. On pods symptomatic for southern stem rot, 45% of the pegs broke proximal to the point of pod attachment. On pods symptomatic for tomato spotted wilt, only 3% of pegs broke proximal to the point of pod attachment.
Late leaf spot and southern stem rot infection levels were both significant in 2003 when late leaf spot caused 79% defoliation of the nontreated check and southern stem rot was symptomatic on 19% of the untreated check (
In 2004 late leaf spot caused 87% defoliation of the nontreated check (
Although the tested fungicide programs were highly effective in preventing pod loss from foliar disease or a combination of foliar and soilborne diseases under severe infection conditions, there was no evidence that any of the fungicide treatments increased peg strength in the absence of disease. Even on over-mature pods with inherently weaker pegs, there was no indication that fungicide treatment enhanced peg strength. Some fungicide programs did increase the peg strength of pods which were symptomatic for southern stem rot, presumably because fungicide treatments efficacious against southern stem rot would slow disease progress on infected pods. In addition, pods symptomatic for southern stem rot were much more difficult to find in plots treated with fungicides efficacious against this disease. Therefore the diseased pods selected for testing from these plots may have had less obvious or advanced symptoms. The relatively poor efficacy of tebuconazole (Folicur) against late leaf spot was representative of tebuconazole performance in replicated experiments and S. C. grower fields in 2003 and 2004. Tebuconazole has previously been consistently effective against late leaf spot since becoming available for grower use in 1994. Subsequent field tests in 2005 confirmed that tebuconazole is no longer effective against late leaf spot in South Carolina.
Our data indicate that for NC-V11 cultivar there was no decline in peg tensile strength as pods matured through the dark yellow to black mesocarp stages. In fact peg strength values for fully mature black pods were numerically greater than those of less mature pods. Thus on healthy plants and assuming equivalent digging conditions (soil moisture), there would seem to be little risk of increased harvest loss until the first over-mature pods with visibly deteriorated pegs are present. This is counter to the commonly held assumption that pegs of mature pods (black mesocarp) are inherently weaker than pegs of less mature pods. Our results are in contrast to
Tomato spotted wilt virus causes significant yield reduction in peanut due to severe plant stunting and pod deformity (
Our measures of peg strength were relatively low compared to previous studies.
Our observation that pegs of healthy pods typically broke at the point of pod attachment is consistent with previous results (
In conclusion, our results demonstrate that growers should make fungicide treatment decisions based on disease prevention efficacy rather than on the assumption of any additional physiological peg strengthening benefits. These results may also prove useful in refinement of harvest-timing guidelines based on the distribution of pod maturity as defined by mesocarp color categories. Specifically, the data indicate that given equivalent digging conditions, pod loss risk will not increase until some over-mature pods with deteriorated pegs are present. However, further research is needed to measure the relationship between peg strength and pod maturity in other commercial varieties.
We appreciate the support of South Carolina peanut growers and the National Peanut Board. This is Technical Contribution No. 5109 of the Clemson University Experiment Station, and is based on work supported by the CSREES/USDA, under project number SC1700191.
1Department of Entomology, Soils, and Plant Sciences, Clemson University, Edisto REC, 64 Research Road, Blackville, SC 29817, USA.