David F. Ritchie

Resistance to bacterial spot in bell pepper induced by acibenzolar-S-methyl

Bell pepper plants sprayed with the chemical acibenzolar-S-methyl (ABM, Actigard 50 WG) showed resistance to subsequent infections with the bacterial spot agent Xanthomonas axonopodis pv. vesicatoria. Induction of resistance was independent of the cultivar used, and was expressed as early as 3 days after treatment and continued for at least 2 weeks. In the field, applications of ABM every 2 weeks, alone or in combination with copper, resulted in disease control similar to the standard treatment of copper plus maneb. Yield response was variable, with certain combinations of chemical treatments and cultivars producing yields as large as the copper plus maneb treatment. In contrast, weekly applications during the entire crop season had a negative impact on yield. In plots maintained free of bacterial spot, applications of ABM every 2 weeks caused a reduction in yield for one cultivar of six tested. The use of chemical inducers for the control of bacterial spot on bell pepper, while generally promising, may result in an unpredictable loss in fruit yield.

Development of Bacterial Spot on Near-Isogenic Lines of Bell Pepper Carrying Gene Pyramids Composed of Defeated Major Resistance Genes

ABSTRACT Disease severity caused by races 1 through 6 of Xanthomonas campestris pv. vesicatoria on eight near-isogenic lines (isolines) of Early Calwonder (ECW) with three major resistance genes (Bs1, Bs2, and Bs3) in different combinations was evaluated in the greenhouse and field. Strains representing races 1, 3, 4, and 6 caused similar high levels of disease severity, followed by races 2 and 5 on susceptible ECW. Race 3 caused severe disease on all isolines lacking resistance gene Bs2. Race 4, which defeats Bs1 and Bs2, caused less disease on isoline ECW-12R (carries Bs1 + Bs2), than on isolines ECW, ECW-10R (carries Bs1), and ECW-20R (carries Bs2). Similar results were obtained with race 4 strains in field studies conducted during 1997 and 1998. In greenhouse studies, race 6, which defeats all three major genes, caused less disease on isoline ECW-13R (carries Bs1 + Bs3) and ECW-123R (carries Bs1 + Bs2 + Bs3) than on isolines ECW, ECW-10R, ECW-20R, and ECW-30R (carries Bs3), but not on ECW-23R (carries Bs2 + Bs3). In greenhouse studies with commercial hybrids, strains of races 4 and 6 caused less disease on Boynton Bell (carries Bs1 + Bs2) than on Camelot (carries no known resistance genes), King Arthur (carries Bs1), and X3R Camelot (carries Bs2). Race 6 caused less disease on hybrid R6015 (carries Bs1 + Bs2 + Bs3) and Sentinel (carries Bs1 + Bs3) than on Camelot. Residual effects were not as evident in field studies with race 6 strains. Defeated major resistance genes deployed in specific gene combinations (i.e., gene pyramids) were associated with less area under the disease progress curve than when genes were deployed individually in isolines of ECW or commercial hybrids. Successful management of bacterial spot of pepper is achieved incrementally by integrating multiple tactics. Although there is evidence of residual effects from defeated genes, these effects alone likely will not provide acceptable bacterial spot control in commercial production fields. However, when combined with sanitation practices and a judicious spray program, pyramids of defeated resistance genes may aid in reducing the risk of major losses due to bacterial spot.

Temperature sensitivity of the hypersensitive response of bell pepper to Xanthomonas axonopodis pv. vesicatoria

ABSTRACT When bacterial spot-resistant pepper plants carrying resistance gene Bs2 and infiltrated with incompatible strains of Xanthomonas axonopodis pv. vesicatoria carrying a functional avrBs2 gene (races P1 and P3) were incubated at 32 degrees C, they exhibited an electrolyte leakage and bacterial multiplication pattern in planta similar to that obtained with a compatible strain (race P4) carrying a nonfunctional avrBs2 gene. They also developed disease-like symptoms. Pretreatment of incompatible bacteria at 32 degrees C before infiltration caused a delay in electrolyte leakage less pronounced than that caused by exposing plants to 32 degrees C. Also, plants had to be exposed to 32 degrees C for an hour prior to inoculation to increase symptom expression. These data suggest that the Bs2 gene is temperature sensitive. In other experiments, the avrBs1-Bs1 interaction appeared to be the most heat tolerant and thus the least likely to revert to compatible, whereas the avrBs3-Bs3 interaction had an intermediate sensitivity to elevated temperatures.

Plant Growth Regulator Effects on Bacterial Etiolation of Creeping Bentgrass Putting Green Turf Caused by Acidovorax avenae

Bacterial etiolation, caused by Acidovorax avenae, is a widespread problem in creeping bentgrass putting green turf. The symptoms normally appear as abnormally elongated turfgrass stems and leaves. Observations at multiple field sites suggest the involvement of plant growth regulators (i.e., GA-biosynthesis inhibitors) commonly applied to turf, alluding to a phytohormone imbalance caused by the bacterium. A 2-year field study examined the effects of trinexapac-ethyl, flurprimidol, and paclobutrazol on bacterial etiolation severity caused by A. avenae. Trinexapac-ethyl applied at 0.05 kg a.i. ha-1 every 7 days and 0.10 kg ha-1 every 14 days increased etiolation compared with all other treatments in both years. Flurprimidol and paclobutrazol were not different from the control but high-rate applications caused phytotoxicity that lowered turf quality early in 2014. When the etiolated turfgrass was removed with mowing, turfgrass treated with trinexapac-ethyl exhibited the highest turfgrass quality on most rating dates. Results from this work illustrate that using plant growth regulator materials with different modes of action is a solution to managing creeping bentgrass growth while limiting the potential for bacterial etiolation outbreaks.

First Report of Xanthomonas translucens Causing Etiolation on Creeping Bentgrass Turf in Illinois, Kentucky, and North Carolina

Symptoms of etiolation, which is an abnormal elongation and yellowing of tillers, have been observed on creeping bentgrass [Agrostis stolonifera L. (CBG)] putting greens for decades; however, symptoms are typically transient and non-problematic. Reports of etiolation have become more frequent recently and research supports the involvement of bacteria (1). During stressful summer periods in 2011 and 2012, 62 CBG putting green samples were submitted to the NCSU Turf Clinic exhibiting symptoms of etiolation, chlorosis, and/or general decline. Microscopic examination of stem and leaf tissue often showed bacterial streaming from the xylem tissue. Symptomatic tissue was surface disinfested in sodium hypochlorite (10% Clorox) for 5 min, blotted dry, and rinsed in sterile dH2O. Disinfested tissue was placed in a small drop of sterile dH2O on a glass microscope slide and cut to allow bacteria to stream into the water for 2 min. The resulting bacterial suspension was streaked onto three nutrient agar (NA) plates and incubated at 30°C overnight. Bacterial colonies varied in morphology and those present in the greatest number based on morphology were re-streaked to isolate individual colonies. Bacterial isolates were tentatively identified to species using rDNA sequencing of 16S and ITS regions (3). Sequencing results showed isolates obtained from 6 locations (in Illinois, Kentucky, and North Carolina) having a positive match (≥99% 16S and ≥93% ITS) to Xanthomonas translucens (GenBank accessions AY572961, HM181927, JX976312, AY253329, and AB680445). Additional research is needed to confirm pathovar designation as X. translucens isolates were similar to both poae and graminis pathovars. A representative isolate (LW10-12A) was also examined for carbon source utilization using the BIOLOG 3rd Gen Microplate (Biolog Inc., Hayward, CA) resulting in a positive identification of X. translucens. Isolate LW10-12A was used to inoculate 6-week-old seeded creeping bentgrass cv. A1 plants maintained at 1 cm height in 3.5 cm diameter containers. Scissors were dipped in a cell suspension (~109 CFU ml-1 in sterile dH2O) and used to cut healthy CBG plants at 1 cm height and the remaining suspension was applied to the foliage until runoff using an atomizer bottle. Non-inoculated plants were cut and misted using sterile dH2O. After inoculation, plants were placed in a sealed clear plastic Camwear container (Cambro Co., Huntington Beach, CA) for 48 h and then transferred to the growth chamber bench (30°C) receiving irrigation twice daily with dH2O. Etiolation was rated within each of the four replicates by counting the number of etiolated leaves that were easily observed as significantly higher than the rest of the turf canopy. Plants inoculated with X. translucens exhibited etiolation of the youngest leaf within 48 h, whereas the non-inoculated plants did not. Symptoms were similar to observations in the field, as etiolated leaves were chlorotic and easily extracted from the turf surface. Microscopic examination showed bacterial streaming and identification of bacteria, using the previously described methods, was positive for X. translucens. Etiolation symptoms persisted over multiple weeks, but a decline in turf quality was not observed. Etiolation has been previously suggested as a precursor to bacterial wilt, caused by X. translucens pv. poae, on annual bluegrass [Poa annua L. f. reptans (Hausskn) T. Koyama] (2) and Acidovorax avenae has also been shown to produce etiolation on CBG (1). To our knowledge, this is the first confirmation of X. translucens as a cause of etiolation in CBG. References: (1) P. R. Giordano et al. Plant Dis. 96:1736, 2012. (2) N. A. Mitkowski et al. Plant Dis. 89:469, 2005. (3) N. W. Schaad et al. Lab. Guide for Ident. of Plant Path Bac., 2001.

Systemic Acquired Resistance Delays Race Shifts to Major Resistance Genes in Bell Pepper

ABSTRACT The lack of durability of host plant disease resistance is a major problem in disease control. Genotype-specific resistance that involves major resistance (R) genes is especially prone to failure. The compatible (i.e., disease) host-pathogen interaction with systemic acquired resistance (SAR) has been studied extensively, but the incompatible (i.e., resistant) interaction less so. Using the pepper-bacterial spot (causal agent, Xanthomonas axonopodis pv. vesicatoria) pathosystem, we examined the effect of SAR in reducing the occurrence of race-change mutants that defeat R genes in laboratory, greenhouse, and field experiments. Pepper plants carrying one or more R genes were sprayed with the plant defense activator acibenzolar-S-methyl (ASM) and challenged with incompatible strains of the pathogen. In the greenhouse, disease lesions first were observed 3 weeks after inoculation. ASM-treated plants carrying a major R gene had significantly fewer lesions caused by both the incompatible (i.e., hypersensitive) and compatible (i.e., disease) responses than occurred on nonsprayed plants. Bacteria isolated from the disease lesions were confirmed to be race-change mutants. In field experiments, there was a delay in the detection of race-change mutants and a reduction in disease severity. Decreased disease severity was associated with a reduction in the number of race-change mutants and the suppression of disease caused by the race-change mutants. This suggests a possible mechanism related to a decrease in the pathogen population size, which subsequently reduces the number of race-change mutants for the selection pressure of R genes. Thus, inducers of SAR are potentially useful for increasing the durability of genotype-specific resistance conferred by major R genes.

Improved peach tree longevity with use of fenamiphos in peach tree short-life locations

Most rates and application schedules of fenamiphos improved tree longevity (P=0.05) compared with no postplant treatment. In two experiments, only one of 28 nontreated trees was productive by the ninth year, while several of the fenamiphos treatments resulted in 100% productive trees. In a third experiment, the percent of productive trees depended upon the preplant treatment and the length of the interval before postplant fenamiphos treatments were initiated. (…)