Common bacterial blight (CBB), incited by the bacterial pathogen Xanthomonas axonopodis pv. phaseoli (Smith) Vauterin et al., is one of the most serious seed-borne diseases of common bean (Phaseolus vulgaris L.) worldwide. Although previous attempts to breed CBB-resistant bean cultivars have had limited success due to the quantitative nature of resistance, progress in marker-assisted selection has created new opportunities for breeders to improve the selection process. The potential of independent linked markers for indirect selection of CBB resistance was evaluated in field experiments in East Lansing and Saginaw, Mich. The presence of the SU91 marker was correlated with lower CBB leaf scores in East Lansing (r = −0.50***) and Saginaw (r = −0.59***) and correlated with pod resistance in Saginaw (r = −0.48***). SU91 exhibited a slight negative correlation with yield (r = −0.20*) in East Lansing but showed no association with yield in Saginaw. Plant selections carrying SU91 were crossed with a different source of CBB resistance linked to the sequence-characterized amplified region marker BC420 on bean linkage group B6. The effects of SU91 and BC420 markers were examined in two greenhouse studies. The presence of SU91 was correlated with lower CBB disease ratings for leaves (r = −0.20*) and pods (r = −0.27***). Presence of BC420 was only correlated with low pod ratings (r = −0.19*) in one experiment and CBB leaf resistance (r = −0.18*) in the second experiment. Presence of both markers resulted in lower levels of CBB resistance than provided by either marker alone, possibly indicating epistatic interactions between the independent loci conditioning CBB resistance in common bean.
Patrick D. O'Boyle, James D. Kelly and William W. Kirk
Joseph C. Kuhl, Kelly Zarka, Joseph Coombs, William W. Kirk and David S. Douches
Late blight of potato (Solanum tuberosum L.), incited by Phytophthora infestans (Mont.) de Bary, is a devastating disease affecting tuber yield and storage. Recent work has isolated a resistance gene, RB, from the wild species Solanum bulbocastanum Dun. Earlier work in Toluca, Mexico, observed significant levels of field resistance under intense disease pressure in a somatic hybrid containing RB. In this study, five transgenic RB lines were recovered from the late blight susceptible line MSE149-5Y, from the Michigan State University (MSU) potato breeding program. Transgenic lines were molecularly characterized for the RB transgene, RB transcript, and insertion number of the kanamycin resistance gene NPTII. Transgenic lines and the parent line were evaluated for resistance in field and laboratory tests. Molecular characterization alone did not predict which lines were resistant. Three of the RB transformed MSE149-5Y lines showed increased resistance under field conditions at MSU and increased resistance in detached leaf evaluations using multiple isolates individually (US-1, US-1.7, US-8, US-10, and US-14). Transfer of RB into late blight susceptible and resistant lines could provide increased protection to potato late blight. The use of the RB gene for transformation in this way creates a partially cisgenic event in potato because the gene's native promoter and terminator are used. This type of transformation provides a chance to generate greater public acceptance of engineered approaches to trait introgression in food crops.
Kathleen M. Baker, William W. Kirk, Jeffrey M. Stein and Jeffrey A. Andresen
Concern in the agricultural community over observed and projected climate change has prompted numerous studies on the possible implications for crop yields. However, relatively little work has focused on disease management. In the upper Great Lakes region of the United States, late blight (Phytophthora infestans) of potato (Solanum tuberosum) is a temporally sporadic disease, occurring only when microclimate conditions within the canopy are favorable and inoculum is present. This and other studies indicate that historical climatological trends in the upper Great Lakes region have resulted in warmer and wetter growing season conditions, as well as local increases in precipitation totals and in the frequency of days with precipitation. Consequently, the risk of potato late blight is increasing. Historical trends in hourly weather variables and potato late blight risk as expressed by a modified Wallin disease severity value index were analyzed at seven regional weather stations from 1948–99. All sites showed significant trends in at least one of the risk estimates. While late blight risk was greatest at all locations in August, periods of increasing risk occurred across the region particularly during July. The increases in disease risk appeared to be associated with upward trends in dry bulb and dew point temperature at nearly all of the stations, especially during July and August. Increased risk of potato late blight has implications for extension agents and commercial horticulturists that include increased emphasis on grower education and application of integrated disease management techniques.