A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss. Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops. A recent example of a successful disease-monitoring program for an economically important crop is the soybean rust (SBR) monitoring effort within North America. SBR, caused by the fungus Phakopsora pachyrhizi, was first identified in the continental United States in November 2004. SBR causes moderate to severe yield losses globally. The fungus produces foliar lesions on soybean (Glycine max) and other legume hosts. P. pachyrhizi diverts nutrients from the host to its own growth and reproduction. The lesions also reduce photosynthetic area. Uredinia rupture the host epidermis and diminish stomatal regulation of transpiration to cause tissue desiccation and premature defoliation. Severe soybean yield losses can occur if plants defoliate during the mid-reproductive growth stages. The rapid response to the threat of SBR in North America resulted in an unprecedented amount of information dissemination and the development of a real-time, publicly available monitoring and prediction system known as the Soybean Rust-Pest Information Platform for Extension and Education (SBR-PIPE). The objectives of this article are (i) to highlight the successful response effort to SBR in North America, and (ii) to introduce researchers to the quantity and type of data generated by SBR-PIPE. Data from this system may now be used to answer questions about the biology, ecology, and epidemiology of an important pathogen and disease of soybean.

Importance of Fungicide Seed Treatment and Environment on Seedling Diseases of Cotton

The importance of fungicide seed treatments on cotton was examined using a series of standardized fungicide trials from 1993 to 2004. Fungicide seed treatments increased stands over those from seed not treated with fungicides in 119 of 211 trials. Metalaxyl increased stands compared to nontreated seed in 40 of 119 trials having significant fungicide responses, demonstrating the importance of Pythium spp. on stand establishment. Similarly, PCNB seed treatment increased stands compared to nontreated seed for 44 of 119 trials with a significant response, indicating the importance of Rhizoctonia solani in stand losses. Benefits from the use of newer seed treatment chemistries, azoxystrobin and triazoles, were demonstrated by comparison with a historic standard seed treatment, carboxin + PCNB + metalaxyl. Little to no stand improvement was found when minimal soil temperatures averaged 25°C the first 3 days after planting. Stand losses due to seedling pathogens increased dramatically as minimal soil temperatures decreased to 12°C and rainfall increased. The importance of Pythium increased dramatically as minimal soil temperature decreased and rainfall increased, while the importance of R. solani was not affected greatly by planting environment. These multi-year data support the widespread use of seed treatment fungicides for the control of the seedling disease complex on cotton.

First Report of Cylindrocladium Black Rot (C. parasiticum) on Partridgepea and Sicklepod

Partridgepea (Cassia fasciculata Michx.) is grown in the southeastern U.S. in food plots for game birds. In 1997, numerous dead plants were observed in a commercial planting for seed production. Perithecia of Calonectria ilicicola Boedijin & Reitsma (imperfect stage: Cylindrocladium parasiticum Crous, Wingfield & Alfenas), a serious pathogen of peanut (Arachis hypogaea L.), were found on the crown of diseased plants. Two isolates each from partridgepea and peanut were grown on potato dextrose agar for 5 weeks. Microsclerotia produced were added to a 2:1 mixture of pasteurized field soil and Pro-mix potting medium (25 microsclerotia per g of mix). Both infested and noninfested potting mixes were put in Super Cell Cone-Tainers (Stuewe & Sons, Corvallis, OR) in the greenhouse and planted to one pre-germinated seed each of either peanut or partridgepea (10 replications). Soil moisture was kept at field capacity and after 7 weeks root rot severity (0 to 4 scale with 4 = dead plant) and fresh weight of whole plants and roots were determined. Mean disease ratings for peanut were 2.1 and 2.5 with the peanut and partridgepea isolates, respectively, and 0.1 for the controls. Mean disease ratings for partridgepea were 3.2 and 3.2 with the peanut and partridgepea isolates, respectively, and 1.0 for the controls. Peanut and partridgepea plant weights were reduced by 50 and 68%, respectively, compared with controls. Reductions in root weights were similar to those for whole plants. The pathogen was consistently recovered from diseased roots. In summary, all four isolates were pathogenic to both hosts, but partridgepea was more susceptible (P ≤ 0.05) than peanut to C. parasiticum. Sicklepod (Senna obtusifolia (L.) H. Irwin & Barneby), one of the most troublesome weeds in the southeastern U.S., was previously observed to have black rot symptoms and perithecia of C. ilicicola on the crowns of diseased plants. Plants grown in soil infested with an isolate of the fungus from sicklepod exhibited typical symptoms and the pathogen was reisolated from diseased tissue. Rotation with soybean (Glycine max L.) traditionally has been the major concern for peanut production in fields with a history of Cylindrocladium black rot; however, these additional hosts also should be considered.

Sensitivity of Isolates of Sclerotium rolfsii from Peanut in Georgia to Selected Fungicides

The fungicide sensitivity of more than 450 isolates of Sclerotium rolfsii from 11 different peanut fields in Georgia was determined based on percent inhibition of mycelial growth on agar amended with tebuconazole, flutolanil, or PCNB. The 11 locations represented a wide geographic distribution and variety of exposure histories to tebuconazole, flutolanil, and PCNB. Most of the populations sampled were significantly more sensitive than the populations that had the longest exposure to the fungicides. Of the three fungicides tested, tebuconazole and flutolanil demonstrated the strongest positive correlation in 1994 and 1995. The differences in sensitivity among locations suggest that fungicide sensitivity among S. rolfsii populations varies across Georgia. The location with the longest exposure history had the lowest sensitivity to all three fungicides.

Frequency and geographical distribution of plant-parasitic nematodes on cotton in georgia

A survey was conducted to examine the geographical distribution of plant-parasitic nematodes in Georgia cotton fields. A total of 778 fields in 11 Georgia counties were sampled from 1 September through 15 December 1995. Four nematode genera parasitic on cotton were found in this survey: Meloidogyne spp., Rotylenchulus sp., Hoplolaimus sp., and Belonolaimus sp. Meloidogyne spp. was present in 9% to 56% of the fields in individual counties. Rotylenchulus sp. was found in 10 counties, Hoplolaimus sp. was found in 6 counties, and Belonolaimus sp. was found in 2 counties. From all of the samples collected for this survey, Meloidogyne spp. were found in 31% of the samples, Rotylenchulus sp. was found in 14%, Hoplolaimus sp. was found in 7%, and Belonolaimus sp. was found in 0.3%. Burke County had the greatest number of fields infested by at least one of these genera (67%) and the greatest number of fields above Georgia's action thresholds (38%). Laurens County had the fewest fields where these genera were present (13%), and only 3% of fields had nematode populations above threshold levels. Data from samples collected from cotton fields and submitted by county agents from 1993 through 1994 were compiled to provide historical information about nematode distribution and population density. The results from this survey show that the major nematodes damaging to cotton are not present in all counties in Georgia. Counties in which cotton has historically been a major crop are likely to have higher levels of Meloidogyne spp., Hoplolaimus sp., and Rotylenchulus sp. in current cotton crops. Counties in which soybean has historically been a major crop are likely to have higher levels of Hoplolaimus sp. and Rotylenchulus sp. in current cotton crops.

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

Field experiments were conducted in one location in 1993 and two locations in 1994 to determine the effects of propiconazole and chlorothalonil tank mix combinations on severity of early (Cercospora arachidicola) and late (Cercosporidium personatum) leaf spot diseases of peanut (Arachis hypogaea L.). In all tests, 10 treatments consisted of 0 and 63 g a.i./ha of propiconazole and 0, 0.315, 0.63, 0.945, and 1.26 kg a.i./ha of chlorothalonil arranged factorially. In 1993, final leaf spot intensity ratings decreased according to nonlinear quadratic functions of chlorothalonil concentrations applied with and without propiconazole. No improvement in leaf spot control was evident with the addition of more than 0.945 kg a.i./ha of chlorothalonil with 63 g a.i./ha of propiconazole. In 1994, conditions were more conducive for leaf spot development. At the Plains location, final leaf spot intensity ratings decreased according to non-linear quadratic functions of chlorothalonil concentrations alone. Leaf spot intensity ratings decreased linearly with increasing rates of chlorothalonil when applied with 63 g a.i./ha of propiconazole. At Tifton, final leaf spot intensity ratings decreased linearly with increasing rates of chlorothalonil with or without propiconazole. Leaf spot intensity ratings were lower on plants treated with tank mixes of chlorothalonil and propiconazole compared to those treated with chlorothalonil alone. Pod yields increased linearly or according to quadratic functions of rates of chlorothalonil with or without propiconazole in both years and all locations. Across all rates of chlorothalonil, yields were higher from plants treated with propiconazole than those treated with the respective rates of chlorothalonil alone.