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Field studies were conducted in Florida to determine the effect of early shoot pruning on the severity of bacterial spot, and on the growth and yield of different tomato (Solanum lycopersicum) cultivars. Two tomato cultivars, two inoculation regimes of bacterial spot pathogen (Xanthomonas perforans), and three shoot pruning programs were arranged in a split-split plot design. The tomato cultivars were Tygress and Security-28; shoot pruning included none, light, and heavy; and X. perforans treatments consisted of non-inoculated plots and plots inoculated with a suspension of the pathogen. Tomato plant height was not influenced by any of the three factors or their interactions, whereas the disease severity was higher in inoculated plots versus non-inoculated plants. Early extra-large fruit weight was affected by tomato cultivars and the inoculation with the bacterial spot pathogen, but not by pruning programs or the interaction among factors. Tomato plants inoculated with X. perforans reduced their extra-large fruit weight by 31% in comparison with non-inoculated plants. There were no differences on early marketable fruit weight among the combinations of each cultivar and the three pruning programs. All three factors individually influenced the seasonal marketable fruit weight of tomato, with no difference between light-pruned plants and the non-pruned control for seasonal marketable fruit weight, regardless of tomato cultivars. However, heavy pruning did reduce seasonal yields by 10% in comparison with the non-pruned control. These results indicated that light shoot pruning, which is the standard grower practice in Florida, did not improve bacterial spot control or tomato yields of total and extra-large marketable fruit, which might save up to $50/acre in reduced labor costs for Florida tomato growers.
Bacterial spot of tomato (Solanum lycopersicum), caused by several Xanthomonas species, is one of the most important diseases of the crop in humid production regions of the world. Conventional breeding approaches for resistance to bacterial spot previously identified race-specific resistances, but current efforts also seek to use quantitative trait loci (QTLs) effecting broad-spectrum resistance. Resistance QTLs and candidate QTLs have been reported on several chromosomes, including a major QTL on chromosome 11. Fusarium wilt (Fusarium oxysporum f. sp. lycopersici) race 3 resistance gene, I-3, is associated with smaller fruit size and has been implicated in other associations with negative characteristics. We evaluated four F2 populations involving the bacterial spot-tolerant breeding lines Fla. 8517, Fla. 8233, and Fla. 8326 across two field seasons to validate and quantify previously identified loci and to test for an effect of I-3 on bacterial spot sensitivity. The chromosome 11 QTL and the I-3 locus were each consistently positively and negatively associated with resistance, respectively, and together explained from 44% to 47% of the variation in each population. The chromosome 11 QTL displayed a dominant to incompletely dominant effect, reducing infection by 14% to 25%. This QTL is distinct from the X. perforans race T3 hypersensitivity loci, Rx-4 and Xv3. The I-3 locus contributed to as much as a 20% increase in infection in I-3/I-3 plants vs. i-3/i-3 plants, and heterozygosity for I-3 generally resulted in an intermediate susceptible response. Significant effects for QTLs on chromosomes 3, 5, and 12 were also observed, but these effects were not consistent in all populations or seasons in which they were segregating. Implications of these findings toward breeding strategies are discussed.
Verticillium wilt of lettuce caused by Verticillium dahliae can cause severe economic damage to lettuce producers. The pathogen exists as two races (Races 1 and 2) in lettuce, and complete resistance to Race 1 is known. Resistance to Race 2 isolates has not been reported, and production of Race 1-resistant cultivars will likely increase the frequency of Race 2 strains. The objective of this research was to select lettuce accessions for resistance to Race 2 isolates of V. dahliae. Two independent populations totaling 314 randomly sampled PIs were evaluated for Verticillium wilt disease incidence (DI) caused by V. dahliae isolate VdLs17 in one unreplicated and two replicated greenhouse experiments. Selection for PIs with reduced DI was conducted between each experiment and plant stems were plated on semiselective media to identify colonized plants that remained non-symptomatic. No accession with complete resistance was identified, although accessions with partial resistance were selected. Genetic variation for the frequency of V. dahliae-colonized plants that remain symptomless was detected. Four PIs (169511, 171674, 204707, and 226641) were selected for further testing in three replicated greenhouse experiments and demonstrated significantly lower disease incidence than the susceptible control cultivars. The results indicate that lettuce has genetic variation for partial resistance to a Race 2 isolate of V. dahliae. The resistant PIs selected in this research are morphologically diverse, and no dependence between rate of bolting and resistance was found. PIs with partial resistance may be useful for breeding lettuce cultivars with resistance to Race 2 isolates of V. dahliae.
Studies were conducted at three locations in Florida to evaluate the effects of kasugamycin alone, in alternation, or as a tank-mix partner with copper bactericides and other fungicides against bacterial spot of tomato. In greenhouse trials, kasugamycin, formulated as Kasumin® 2L, reduced bacterial spot severity by up to 37.5% compared with a non-treated control. Little advantage in disease control was observed by mixing kasugamycin with other fungicides. Kasugamycin was assessed in six field trials. In the four field trials that tested kasugamycin alone, it was as effective as the standard copper + mancozeb treatment for the control of bacterial spot. In four trials, no benefit was observed in applying kasugamycin as a mixture with copper + mancozeb, and only one of three trials did alternating kasugamycin with copper + mancozeb improve bacterial spot control over either the copper + mancozeb standard or kasugamycin alone. Although kasugamycin was effective for the control of bacterial spot in greenhouse and field trials, rapid development of resistance in field populations of X. perforans may shorten the effective use of this antibiotic.