Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians, is a devastating disease of lettuce worldwide. Because there are no chemicals available for effective control of the disease, host-plant resistance is highly desirable to protect lettuce production. A new method for fast screening and accurate identification of bacterial leaf spot (BLS)-resistant lettuce has been developed in our laboratory. A total of 79 lettuce genotypes (69 germplasm lines and 10 adapted cultivars) were evaluated with this technique for response to X. c. vitians. Disease incidences ranged from 92% to 100% and disease severities were between 1.6 and 3.6 on the 0 to 4 scale. No highly resistant genotypes were identified. However, 12 genotypes did not significantly differ for disease severities from the moderately resistant ‘Little Gem’ lettuce that was used as a resistant control. Comparison of disease severities of 10 commercial cultivars and three moderately resistant germplasm lines tested at the seedling stage and adult stage showed a high positive correlation (r = 0.87, P < 0.0001) between tests. The new screening method should be useful in breeding programs, in which great numbers of plants need to be tested during germplasm evaluation, and for single plant selection as well as other studies. The identification of new sources of moderate resistance in this study could facilitate development of cultivars with a higher level of resistance through the gene pyramiding approach.
Huangjun Lu and Richard Raid
Huangjun Lu, Alan L. Wright and David Sui
Banded cucumber beetle [BCB (Diabrotica balteata)], serpentine leafminer [SL (Liriomyza trifolii)], and aphids (Homoptera: Aphididae) are among the major insect pests that cause significant economic damage to lettuce (Lactuca sativa) in southern Florida. Four romaine cultivars and three iceberg cultivars, currently used in Florida's lettuce production, were evaluated from Oct. 2010 to Jan. 2011 in separate field experiments for their responses to insect infestation. Lettuce cultivars differed significantly in their responses to infestations of BCB and aphids. Cultivar 70096 had the lowest percent (3.7%) of BCB feeding damage among the romaine cultivars. Romaine cultivar Manatee also had significantly lower BCB feeding damage (12.1%) than the susceptible cultivars Okeechobee (19.8%) and Terrapin (19.1%). The lowest level of infestation of aphids was observed on ‘Manatee’, followed by ‘70096’, whereas ‘Okeechobee’ and ‘Terrapin’ had severe infestation of aphids. The iceberg cultivars were similar to one another in their responses to BCB and SL but not in their response to aphid infestation. Yield was decreased from 3% to 37% for six of the seven cultivars grown under the adverse environmental conditions of insect infestations and cold weather in Dec. 2010, but the yield of ‘70096’ did not decline. The cultivars identified as resistant to insects can play an important role in integrated pest management (IPM), which may decrease pesticide application, reduce production costs, and protect the environment.
Long Ma, Kevin E. Kenworthy, Huangjun Lu and Ronald Cherry
Common carpetgrass is a warm-season grass species that is commonly used as a pasture grass in Louisiana and as an alternative low-maintenance lawn grass in the southern Unites States. Understanding genetic variation for traits related to seed production is important to determine breeding strategies for improvement. Ten genotypes were analyzed for number of branches per inflorescence, number of spikelets per branch, and percentage seed set under self-pollination and open pollination. The 10 genotypes exhibited a wide range of variation for number of spikelets per branch and seed set but had a narrow range of variation for number of branches per inflorescence. Genotype was more important than year in contributing to number of branches per inflorescence, whereas the year variance component had a greater impact on number of spikelets per branch. The relative importance of genotype and environment for seed set differed between the two modes of pollination. Broad-sense heritability was 0.35 for number of branches per inflorescence and 0.07 for number of spikelets per branch. Heritability of seed set was 0.29 when the inflorescences were selfed and 0.50 when the inflorescences were subjected to open pollination. Average percentage seed set of the 10 genotypes under self-pollination and open pollination was not significantly different. However, two of 10 genotypes had significantly different means for seed set between self-pollination and open pollination. This information should be useful to plant breeders to select appropriate breeding methods for cultivar development of common carpetgrass.
Yunwen Wang, Huangjun Lu, Richard N. Raid, Gregg S. Nuessly and Georgy Faroutine
Bacterial leaf spot (BLS) disease, caused by Xanthomonas campestris pv. vitians (Xcv) has become an increasingly damaging disease in the lettuce production areas of the United States. To understand the nature of the outbreaks of this disease, the pathogenic variations for causing disease were evaluated on 29 lettuce cultivars and germplasm lines using three Xcv isolates recovered in different years from the Everglades Agriculture Area (EAA) of Florida. Significant differences were shown in both the BLS incidences and disease severities among the three Xcv isolates, and the rank from high virulence to less severity was L7 > JF196 > NF1. Our results suggest that the pathogenic variations of the isolates may have been associated with the epidemic outbreaks of BLS in EAA. Among the 29 lettuce genotypes, the host plant resistance was characterized by specific host genotype and Xcv isolate interactions. The leaf lettuce PI358001-1 was consistently in the high resistant category to all three Xcv isolates, and is a promising resistant source for development of resistant cultivars.