Efficacy of Seed Treatments, Microbial and Biochemical Pesticides for Managing Early Tan Spot and Stripe Rust Diseases of Wheat

Wheat is a major cereal crop in the U.S.A and in the world. However, its production is hampered by a number of factors both biotic and abiotic. Foliar diseases like tan spot caused by Pyrenophora tritici-repentis and stripe rust caused by Puccinia striiformis f. sp. tritici, are economically important diseases causing over 5-55 % yield loss. To assess the efficacy of fungicide seed treatments in the management of early tan spot, two hard red spring wheat cultivars “Select” and “Brick” were treated with seedapplied commercially available combo fungicides metalaxyl + pyraclostrobin + triticonazole and difenoconazole + mefenoxam + sedaxane + thiomexotham. A nontreated check consisting of naked seed for both cultivars was included. The treated and non-treated seeds planted in the greenhouse and inoculated with P. tritici-repentis at 7, 14, 21 and 28 days after planting (DAP). Disease severity, lesion size, and number of lesions ratings were taken from inoculated plants at 7 and 14 days after inoculation (DAI) for each inoculation period. To evaluate the efficacy of fungicide seed treatments on stripe rust, two hard red winter wheat cultivars “Expedition” (moderately susceptible) and “Alice” (susceptible) to stripe rust were seed treated with two commercial fungicides pyraclostrobin, and difenoconazole + mefenoxam and planted in cones and then transferred to a growth chamber at 10°C. A non-treated seed check was included for comparison. The plants were inoculated with Puccinia striiformis f. sp. tritici obtained from a wheat field near Brookings the previous season. Ratings for stripe rust severity were done at 20 days after inoculation for the plants inoculated at 2 weeks after planting. To test the effect of seed treatment on early tan spot development under field conditions, field trials were conducted from the South Dakota State University (SDSU) Northeast Research Farm near South shore in Watertown and at SDSU Volga Research Farm with seed treatments having active ingredients; prothioconazole+ penflufen+ metalaxyl; sedaxane; pyraclostrobin; metalaxyl + ipconazole; difenoconazole + mefenoxam and the untreated seeds as the check. Results from the greenhouse seed treatment control of early tan spot development indicated a significant difference among seed treatments with a low tan spot severity in the treated plants. Untreated plants for both cultivars had the highest percentage disease severity ranging from 50- 70 %, large lesions (0.5-0.6 centimeters) and high lesion counts (30-40) at 1, 2, 3 and 4 weeks after planting compared to the treated pots that had significantly low percentage disease severity, lesion numbers, and lesion size. Similarly, significantly lower tan spot severity was observed in the seed treated plots compared to non-treated in the field trial at both locations. Plots with fungicide seed treatments had higher numbers of plants/m2 compared to the non-treated. Winter survival was significantly high in the treated plots than the untreated for both locations. Likewise, there was significantly a higher grain yield for plots treated with fungicides than the untreated plots. To assess the efficacy of the biochemical and microbial pesticides, a hard red spring wheat cultivar “Select” was planted in the cone-tainers. The 3 and 6 weeks old seedlings were pre-treated with products containing active ingredients Bacillus subtilis QST713, Bacillus amyloquafasciens D747, Streptomyces lydicus WYEC 108, hydrogen peroxide + peroxyacetic acid and azadirachtin. Pyraclostrobin acted as a positive control. All greenhouse plants were then inoculated with Ptr three days after initial product application at three and six weeks after planting. The same treatments were used in a field setting and a split-plot design was used for two separate fields, namely; the organic and conventional plots at the Southeast Research Farm. Results from the greenhouse and field study showed significant tan spot disease control in the greenhouse with low tan spot severities in cones treated with pyraclostrobin, Bacillus subtilis QST713, Bacillus amyloquafasciens D747, Streptomyces lydicus Wyec 108. Plant vigor was also significantly increased in terms of greenness with the treated plots having higher green ratings than the untreated. Azadirachtin was not significant at managing the tan spot disease or improving the plant greenness. Likewise, yield was significantly high in plots treated with microbial pesticides than in the control/untreated plots. In a study to establish the sensitivity of Pyrenphora tritici repentis to fungicides, ninety isolates collected from North Dakota, South Dakota, Montana, and Kansas were tested for sensitivity using both spore and mycelia assays on picoxystrobin, prothioconazole + tebuconazole and azoxystrobin + propiconazole. From the mycelia assays, thirty out of the ninety isolates tested in microtiter plate assays showed insensitivity to picoxystrobin half and full rates with and without Salicylhydroxamic acid (SHAM). Further assessments of the five Ptr isolates involving spores showed germination of all isolates on all fungicides with picoxystrobin having the highest percentage spore germination. The double rate of the fungicides had the least germination percentage for all the isolates tested. Spore germination was nullified when the fungicide picoxystrobin was amended with Salicylhydroxamic acid (SHAM) and no spore germination was recorded meaning all the isolates were sensitive to the fungicides and the prior registered insensitivity was a result Ptr evading the cytochrome b site of QoI action in the absence of SHAM. The findings of this study could be an important source of information for growers and the industry in making wheat production more profitable by informed tan spot and stripe rust management decisions