Tank Mix Combinations of Propiconazole and Chlorothalonil for Control of Leaf Spot Diseases of Peanut

Post-infection activities of fungicides against Cercospora arachidicola of peanut (Arachis hypogaea)

BACKGROUND

Despite strong indirect evidence of post-infection activity by a selection of systemic fungicides against Cercospora arachidicola, the causal organism of early leaf spot of peanut, direct post-infection activities in this pathosystem have yet to be reported in detail. This study was conducted to describe the activities of pyraclostrobin, penthiopyrad and prothioconazole on early leaf spot when each fungicide was applied after pathogen penetration began and throughout the incubation period.

RESULTS

Most C. arachidicola penetration events occurred between 3 and 5 days after inoculation (dai), and the mean incubation period was 11.8 dai. Post-infection activities of the systemic fungicides were similar for all dependent variables measured. Systemic fungicides reduced lesion density compared with the non-treated control when applied at 3, 5 and 7 dai, and disease severity was >60% less for leaves treated with a systemic fungicide at all application dates (3, 5, 7, 9, 11 and 13 dai).

CONCLUSIONS

Pyraclostrobin, penthiopyrad and prothioconazole showed similar systemic mobility within peanut leaves and activities against C. arachidicola, and appear to completely arrest the development of the pathogen at least 2 days post penetration, and limit pathogen colonization even when applications occur after symptom onset.

Effects of Reduced Tillage, Resistant Cultivars, and Reduced Fungicide Inputs on Progress of Early Leaf Spot of Peanut (Arachis hypogaea)

Field experiments were conducted in 2000 and 2001 on Georgia Green, Florida MDR-98, and C-99R peanut (Arachis hypogaea) cultivars in Tifton, GA, to determine the effects of tillage practices on early leaf spot (Cercospora arachidicola) epidemics under standard fungicide regimes and fungicide regimes with fewer applications. Leaf spot epidemics were suppressed in reduced tillage (strip-till) plots compared with conventional tillage plots and were suppressed in MDR-98 and C-99R cultivars compared with the standard runner-type cultivar, Georgia Green. Within tillage and cultivar combinations, leaf spot intensity typically was lower in plots treated with fungicides at standard intervals (seven total applications) than in those treated at extended intervals (four total applications). However, in most cases, leaf spot control in extended interval treatments in the strip-till system was comparable to that in the standard interval treatments in conventional tillage. Based on these results, the number of fungicide applications could be reduced without compromising control of leaf spot when reduced tillage is used, especially if combined with moderately resistant cultivars. Suppression of leaf spot epidemics in the strip-till plots did not coincide with higher yields in either year. In 2001, yields were lower in strip-till plots than in conventional tillage plots. Yields were typically higher in the cultivar C-99R than in Georgia Green, regardless of the tillage treatment.

Accumulation and decay of chlorothalonil and selected metabolites in surface soil following foliar application to peanuts

One of the principal uses of the fungicide, chlorothalonil, is control of foliar peanut diseases. Recent assessments indicate its runoff from treated fields in southeastern states presents risks to aquatic life. Two factors that control its runoff are how much reaches soil surfaces and degradation rates. To address these questions and to evaluate accumulation and decay of key metabolites, soil samples (0-2 cm) were collected after seven chlorothalonil applications on experimental peanut plots in south central Georgia during the 1999 growing season. At the start of and during laboratory incubations, samples were analyzed for the parent and degradates by HPLC-PDA-APCI-MS. The maximum observed residue levels were after the second application, after which canopy closure reduced soil deposition from later applications to 5-10% of applied amounts. After the last spray, < 5% of the cumulative chlorothalonil applied was detected in the soil. Foliar interception and dissipation and rapid soil degradation contributed to low residue levels. Soil half-lives were < 1-3.5 days for chlorothalonil and 10-22 days for its principal degradate, 4-hydroxychlorothalonil. Other daughter and granddaughter products were detected, some of which accumulated during the growing season. Results emphasize the plant canopy role in controlling the amount of fungicide sprays that reach soil surfaces and suggest concentration-dependent chlorothalonil degradation with degradation rates increasing as soil loading decreases. The study indicates that the 30-day field half-life often used for risk assessments of this pesticide is too long for one of its most important agronomic uses, i.e., in southeastern peanut production. It also indicates that the principal metabolites are more persistent than the parent, and more study is needed to identify and quantify their fate pathways.