Cover crops and nematicides: comprehensive nematode IPM across the grove landscape

Cover crops and nematicides: comprehensive nematode IPM across the grove landscape

Report Date: 03/25/2021
Project: 18-036C   Year: 2021
Category: Horticultural & Management
Author: Larry Duncan
Sponsor: Citrus Research and Development Foundation

Nematicide effects on nematode populations were measured in December, as previously described. The average number of sting nematodes in aldicarb plots was 10% higher (NS) than that in untreated plots, whereas oxamyl, Syngenta product and Nimitz plots had 34%-42% fewer sting nematodes (P<0.05) than in untreated plots.  Nematode numbers in the plots treated with the other nematicides ranged from 89%-93% of that in untreated plots, none of which were significant differences.  The average cumulative numbers of sting nematodes measured 60 days following each of the four seasonal applications thus far during the project (areas under the curve) were 55%, 54%, 37%, 21%, and 8% lower than those in untreated plots for the Syngenta compound (P=0.05), oxamyl (P=0.05), Nimitz, (P=0.05) Salibro, and Velum Prime, respectively.  Cumulative sting nematodes in aldicarb treated plots were 16% more numerous than in untreated plots.Average fibrous root mass density for all nematicide treatments ranged between 7%-47% greater than for non-treated plots.  Unlike during the previous June, no treatment resulted in significant root increase during the fall period. The growth of the tree trunks during 2 years between February 2019 and January 2021 was 26% greater for trees treated with oxamyl than that of untreated trees (Dunnett, P<0.05). Growth rates of trees treated with other nematicides were between 2-13% greater than the untreated trees, but the differences were not significant. There remained a strong inverse linear relationship (r= -0.75, n=7, P=0.05) between trunk growth during 2 years and the average sting nematode population density (log-transformed) during that time. The harvested fruit in were counted.  The number of oranges per tree in both 2020 and 2021 were very highly correlated (r=0.68 and r=0.64, P=0.001) with trunk cross-sectional area measured at the beginning of the trial (2/2019). Larger trees initially produced more fruit, irrespective of treatment.  The average trunk area differed greatly between treatments at the beginning of the trial, therefore we determined the %increase in number of fruit per treatment between2020 and 2021.  The yield/tree declined by 22% in untreated trees between 2020 and 2021, but increased in all of the nematicide treated plots.  Aldicarb and oxamyl plots increased (P=0.05) by 166% and 126%, respectively while plots treated with combinations of Velum Prime and Nimitz increased by 44% and those treated with combinations of Salibro and the Syngenta product increased by 23%.In a second trial comparing untreated trees to trees treated with aldicarb in April 2019 and 2020, there were no differences in the trunk cross-sectional areas of either treatment at 20 months after the first treatment (untreated=2403 mm2 vs aldicarb=2409 mm2).  Nor was there a significant increase in growth since summer 2020.  There was no treatment effect on the very small number of fruit per tree (10.2 untreated vs 6.2 aldicarb). Although young tree growth in this grove is very uneven, the cause(s) are other than sting nematodes which were below detectable levels in most plots.In the perennial peanut trial, sting nematodes in the native vegetation middles increased to 85 nematodes/250 cc soil from an average of 51 nematodes during the previous 6 months.  However, the populations remained below 10 nematodes in the plots planted with peanut regardless of whether there was some incursion of weeds in some of the peanut plots. Indeed, during the monthly measurement the past 6 months, sting nematodes in peanut middles averaged just 7.7% ± 2.02% (mean and standard error) of those in native vegetation middles.  Unlike the previous spring, oxamyl did not significantly reduce the sting nematodes in the plots during the autumn (just 25% reduction).  Citrus fibrous root density in plots without peanut middles or oxamyl treatment was 0.21 mg/g soil, compared to 28 mg/g in plots treated with oxamyl, 29 mg/g in plots with peanut middles, and 30 mg/g in plots with both peanut and oxamyl.  There was an interaction (P=0.01) between oxymyl and peanut effects on trunk size.  Trees were 14% larger (P<0.1) when peanut rather than native plants were in the row middles.  Trees treated with oxymyl in peanut plots were 26% larger than those not treated (P=0.03), but were not different than untreated trees in plots with weed middles. Trunk size was highly correlated with number of fruit per tree, but there were no significant effects or interactions between fruit number and either treatment.        


Your browser does not support pdfs, click here to download the file.