Relations of Cercospora beticola with Host Plants and Fungal Antagonists
Cerco pora leaf pot (CLS) cau ed by Cercospora beticola Sacc., is sti ll considered to be the mo t important foliar di ease of ugar beel. The di ea e ha been reported wherever ugar beet i grO\\ n (Bieiholder and Weltzien 1972). Since the di ease wa fir t identified. management of CLS of ugar beet has been an ongoing mi ion of plant pathologists. Toda}. everal trategie are available and applied either s ingly or in combination to manage the di ea e. The e management trategie , which were ummarized by Windels et al. (1998), include cultural practice uch a deep tillage, rotation with non-host crops and identification and elimination of econdary weed host . Other include breeding and use of re i.tant cultivars and application of fungicide. The u e of re i tant sugar beet cultivars has long been an integral pan of CLS management; however, problems associated with selection of re i tant cuhivars again t CLS are wel l documented and were recently reviewed by Weiland and Koch (2004). According to the author , resistance to CLS in sugar beet has been de cribed as quantitatively in herited and rate limiting with respect to disease development (Smith and Gaskill 1970, Ro i et a l. 1999). Although resistant cultivars have proven effective in both North America and Europe, they nonetheless exhibit low heritability (Smith and Ruppel 1974), and cultivar bred for Cercospora resistance can still exhibit leaf pots if climatic condition favorable for the disease occur.
Root infection of sugar beet by Cercospora beticola in a climate chamber and in the field
Sugar beet root infection by Cercospora beticola, the causal agent of Cercospora leaf spot (CLS), was studied in a climate chamber and in the field. In the climate chamber, root incubation of susceptible seedlings with a conidial suspension resulted in disease incidences that were significantly different for two sugar beet cultivars (Auris: 0.8 ± 0.14 and A00170: 0.5 ± 0.18; P < 0.05) with regard to the control treatment 35 days after root incubation in a standard potting soil-fine river sand mixture. In a field trial with susceptible cv. Savannah with soil-incorporated CLS-infested leaf material, disease developed four weeks earlier in the infested plots than in the control plots. The probability that disease develops in the field was significantly higher for the infested than for the control plots (P < 0.05). Symptomless plants from infested field plots transferred to the glasshouse to induce leaf spot symptoms showed a significantly higher probability to induce symptom development (0.4 ± 0.08), than plants from control plots (0.02 ± 0.02) (P <0.05) 14 days after transfer. This probability was significantly higher than for plants that remained in three of the infested field plots (0.2 ± 0.04; 0.2 ± 0.05 and 0.2 ± 0.04 respectively), except for one infested field plot (0.4 ± 0.05) on July 5. We conclude that C. beticola is able to infect sugar beet seedlings through their roots and that latent CLS infections in sugar beet lead to symptom development at high temperatures (> 20 °C) and high relative humidity (> 95) in our climate chamber or after canopy closure in the field. Quantification of root infection and long term survival in soil is necessary to assess its contribution to the epidemiology and life cycle of Cercospora beticola. Cultural methods such as a wider crop rotation, management of crop debris and ploughing systems may provide control strategies alternative to or reducing fungicide input.
Effect of Soybean Cyst Nematode (Heterodera glycines) on Yield of Resistant and Susceptible Soybean Cultivars Grown in Ohio.
Soybean (Glycine max) producers in Ohio rarely use soybean cyst nematode (Heterodera glycines, SCN)-resistant cultivars because of concerns over limited yield potential and lack of resistance to Phytophthora sojae. A two-year study was initiated to determine grain yield and nematode population increase on soybean cyst nematode-resistant cultivars in maturity groups II and III in production fields. Sites differed in soil texture, nematode densities, and P. sojae infestation at a number of locations in Ohio. Soil was assayed for nematode densities before planting and at harvest. Yields of resistant cultivars averaged 0% to 18% higher than those of susceptible cultivars in fine-textured soils with average preplant populations ranging from 463 to 14,330 SCN eggs/100 cm(3) soil. In coarse-textured soils, yields of susceptible cultivars were 21% to 56% less than the resistant cultivars with average preplant densities ranging from 1,661 to 15,558 SCN eggs/100 cm(3) soil. The reproductive index ranged from 0.1 to 5.5 for resistant cultivars and 0.4 to 112 for susceptible cultivars. In 1993, yield of P. sojae-susceptible, nematode-resistant 'Asgrow A 3431' was as high as yield of the P. sojae-resistant, nematode-susceptible cultivar 'Resnik' in a Phytophthora-infested field. The nematode-resistant cultivars Madison Experimental 131527 and Asgrow A3431 had higher yields than AgVenture AV1341 and susceptible cultivars Resnik and Kenwood when compared over five nematode-infested sites. Nematode-resistant cultivars were found to be excellent alternatives to currently grown susceptible cultivars for managing SCN where group III cultivars are used. However, better cultivar alternatives may be needed for sites with combined Phytophthora root rot and cyst nematode problems.