RESUMO
Delaware, the eastern shore of Maryland, and southern New Jersey have been the center of lima bean (Phaseolus lunatus L.) production in the eastern United States for nearly 50 years (1). Downy mildew has been the most important disease of lima bean in the humid eastern United States over that period. The causal agent of downy mildew, the oomycete pathogen Phytophthora phaseoli Thaxt., was first identified on lima bean in Connecticut in 1887 by Thaxter. Signs and symptoms of lima bean downy mildew include infection, necrosis and abscission of flowers, and shepherd's crooking of racemes, shoot tips, and petioles (1). Sporangia develop on shoot tips, petioles, pins (small pods), and pods in the field and on hypocotyls in-vitro. Since 2005, approximately 50% of the baby lima beans processed in the United States have been grown in Delaware and the eastern shore of Maryland. In 2008, commercial lima bean production began on the eastern shore of Virginia in Accomack County but no downy mildew was reported in that season. In 2009, approximately 1,825 ha in Accomack and Northampton counties were planted to baby lima bean. Weather conditions in 2009, including above average rainfall, were conducive for the development of downy mildew on the Delmarva Peninsula. Downy mildew was widespread in growers' fields in August and September in butter bean in southern New Jersey and baby lima bean in Sussex County, DE. In August 2009, a home gardener in Rappahannock, VA sent samples of infected lima bean pods from baby, Fordhook, and pole lima bean plants to the Virginia Tech Plant Disease Clinic in Blacksburg. On the basis of morphometric analysis, samples were determined microscopically to be infected by a Phytophthora sp. with rather uniform sporangia averaging 39 × 22 µm and short pedicels, diagnostic for P. phaseoli (1). On October 27, 2009, field scouts in Accomack County, VA identified two lima bean fields planted to cv. C-Elite-Select exhibiting moderate symptoms of downy mildew. Samples were brought to the Plant Diagnostic Clinic at the University of Delaware under USDA-APHIS permit and determined to be P. phaseoli based on morphometric analysis. Samples were inoculated onto a lima bean cultivar differential to determine pathogenicity to complete Koch's postulates and to determine their physiological race. Samples were inoculated onto lima bean cvs. 184-185 and C-Elite-Select, which are susceptible to race F and resistant to race E, Eastland and 8-78, which are susceptible to race E and resistant to race F, and Concentrated Fordhook, susceptible to all known races (1). Three pots containing five emerging seedlings each were inoculated with sporangia (approximately 103 per ml) prepared by soaking infected pods in 500 ml of sterile distilled water for 1 min with gentle agitation. Plants were placed in a Percival dew chamber with intermittent misting and set at 19. Infection and disease development were assessed daily and signs developed 7 days postinoculation in cvs. 184-85, C-Elite-Select, and Concentrated Fordhook, but not in Eastland and 8-78. Cultivar differential tests indicated that the isolates were P. phaseoli race F. Hypocotyls of infected plants were scraped, and isolations made on lima bean pod agar confirmed the presence of P. phaseoli. To our knowledge, this is the first time that downy mildew of lima bean has been reported in Virginia. Reference: (1) T.A. Evans et al. Plant Dis. 91:128, 2007.
RESUMO
Stem rot of peanut, caused by the soilborne fungus Sclerotium rolfsii, is greatly influenced by environmental conditions. Disease management programs rely heavily on fungicides, which are applied on a calendar-based program. To determine whether improved control of stem rot could result from weather-based spray advisories, models were constructed using what is currently known about the biology of S. rolfsii and etiology of stem rot epidemics in peanut. Spray advisories based on soil temperature, precipitation, and host parameters were tested, along with advisories focusing on soil temperature and precipitation or precipitation alone. The advisories were evaluated and compared with the currently used calendar-based program over four locations annually for 3 years. Fungicide application timing had a significant effect on both stem rot control and resulting pod yields. In general, stem rot control following the advisories considering soil temperature, precipitation, and canopy growth was similar or better than that offered by the calendar-based program, but yields generally were comparable. The AU-Pnut advisory for foliar diseases also was effective for scheduling azoxystrobin applications for stem rot.
RESUMO
Tobacco cyst nematode (Globodera tabacum solanacearum) isolates were collected from 11 locations in Virginia, 3 in North Carolina, and 1 in Maryland. Isolates from each location were maintained and increased on flue-cured tobacco, Nicotiana tabacum cv K326. Plants of flue-cured tobacco cultivars K326 (susceptible) and NC567 (resistant) were each inoculated with 6,000 G. t. solanacearum eggs/plant. Tests were conducted over one (6 weeks) or two (14 weeks) generations of the nematode. Shoot and root weights and the number of nematodes present within a 1-g subsample of feeder roots were recorded at completion of the tests. Nematode counts were categorized by nematode life stage (vermiform, swollen, pyriform, and adult). Although significant differences in nematode development were detected among isolates, differences were not consistent across experiments. Results indicate similar virulence among G. t. solanacearum isolates on resistant and susceptible flue-cured tobacco cultivars. Therefore, plant breeders may effectively use a single G. t. solanacearum isolate when screening tobacco germplasm for resistance.
RESUMO
The tobacco cyst nematode (Globodera tabacum solanacearum) continues to pose a serious threat to flue-cured tobacco production in Virginia and nearby states. Soils were sampled from five uninfested and two infested flue-cured tobacco-producing locations. Twenty-three edaphic factors were characterized to determine if any were correlated with G. t. solanacearum reproduction. Comparisons were also made between pasteurized and natural soils to determine if biological suppression of G. t. solanacearum reproduction might be occurring in currently uninfested areas. Differences in G. t. solanacearum reproduction were noted among the soils, but results were inconsistent across the three trials conducted in this study. Only soil pH correlated significantly with nematode reproduction, and then only in one of three trials. Globodera tabacum solanacearum reproduced with similar efficiency in natural and pasteurized soils.
