Journal of Plant Diseases and Protection

Krauthausen, Hermann-Josef; Laun, Norbert; Wohanka, Walter

doi: 10.1007/BF03356375pmid: N/A

Black rot of Brassica spp. caused by the bacterial pathogen Xanthomonas campestris pv. campestris (XCC), is a seedborne disease. Because the pathogen can easily be spread by splashing water, there is a high risk of spreading it by overhead irrigation systems during the raising of cabbage transplants. In glasshouse experiments, the efficacy of different methods to reduce XCC dispersal and subsequent black rot development during the raising of transplants were examined. The three methods tested were 1) chlorine dioxide dosing of overhead irrigation water, 2) spray treatments of transplants with commercially available products that contain disinfectant or have bactericidal properties (benzoic acid and copper hydroxide) and 3) an ebb and flood irrigation system. Trays with inoculated cauliflower transplants that had been raised from modules were placed between trays with healthy transplants. XCC transmission from the spray inoculated infector to non-inoculated transplants was detected using PCR after four days. The mean efficiency of 1–2 sprayings with benzoic acid and copper hydroxide in reducing black rot development was 11.6% and 32.2%, respectively. Overhead irrigation with chlorine dioxide enriched water resulted in a mean efficiency of 93.5%. No symptoms of black rot were observed in the ebb and flow irrigation system. The proportion of symptomless, XCC-DNA positive transplants at the end of the incubation period followed the same pattern as the visual black rot development. To obtain a minimum concentration of 0.21 mg l−1 chlorine dioxide effective against XCC, a feed-in concentration of 3 mg l−1 was used, resulting in a final concentration of 0.5 to 1.0 mg l−1 wetting the leaf surface. The positive effects of both “sub-” and “chlorine dioxide dosed” irrigation to reduce the spread of XCC during transplant raising were still apparent in the field and resulted in a healthier crop.

Weber, Roland; Hahn, Matthias

doi: 10.1007/BF03356376pmid: N/A

A simple test based on the germination of conidia of Botrytis on agar media augmented with various fungicides has been developed. Average concentrations causing a 50% reduction of germ-tube growth (EC50) of highly sensitive isolates were determined on 1% malt extract agar (thiophanate-methyl 0.090 ppm; iprodione 0.566 ppm; fludioxonil 0.026 ppm; fenhexamid 0.144 ppm), 1% malt extract agar with 100 ppm salicyl hydroxamic acid (QoI fungicides, viz. trifloxystrobin 0.009 ppm; pyraclostrobin 0.013 ppm; azoxystrobin 0.087 ppm), 0.5% yeast extract agar (boscalid 0.069 ppm) and 0.5% sucrose agar (cyprodinil 0.053 ppm). In order to detect different levels of resistance against these various fungicides, two discriminatory concentrations were identified for each compound. A routine assay method was developed in which drops of a conidial suspension harvested directly from diseased plant material or sporulating cultures were incubated on each of 20 different agar media. Because of a very short time-span of 24–48 h between sample collection and evaluation of results, field-specific information on the occurrence, frequency and types of resistance of Botrytis against common botryticides in soft-fruit production may be generated prior to the main fungicide spray season at blossom time.

Peneder, Stefan; Koschier, Elisabeth

doi: 10.1007/BF03356377pmid: N/A

Bioassays using bean leaf discs were conducted to investigate toxic and behaviour-modifying effects of carvacrol and thymol, two monoterpenoid phenols, on second instar larvae of Frankliniella occidentalis (Thysanoptera: Thripidae) at concentrations of 0.1% and 1%. We tested the contact toxicity of carvacrol and thymol to F. occidentalis larvae and studied the settlement preference of thrips larvae for treated or untreated bean leaf discs and the effects of the compounds on their feeding activity. Mortality rates after 24 h were 24.1% for carvacrol at 1% concentration, 16.5% for carvacrol at 0.1%, 7.7% for thymol at 1% concentration and 3.6% for thymol at 0.1%. Thrips larvae showed a significant preference for untreated leaf discs in choice tests with carvacrol 1% or thymol 1%. Likewise, in the no-choice tests feeding damage after 24 h was significantly lower on leaf discs treated with 1% carvacrol or 1% thymol compared to the control, but not on discs treated with 0.1% concentration of either compound. We conclude that an application of carvacrol or thymol to bean leaves does not result in a high mortality of F. occidentalis larvae but affects their behaviour.

Massa, Dario; Gerhards, Roland

doi: 10.1007/BF03356378pmid: N/A

In this study, the results of two-year investigations on herbicide resistance in silky bent grass (Apera spica-venti) populations are presented. Whole-plant bioassays were conducted with different herbicides on over 250 A. spica-venti populations from Central and Eastern European agricultural fields where herbicides failed to achieve satisfactory control. Results showed that over 60% of the suspected populations could be rated resistant to acetolactate synthase (ALS)-inhibitors, resistance to acetyl-CoA (ACCase)-inhibitors could be observed in only a few cases and no resistance to photosystem II (PSII)-inhibitors was detected. Dose-response experiments conducted in the greenhouse on resistant populations with the herbicides flupyrsulfuron-methyl, mesosulfuron+iodosulfuron and fenoxaprop-P-ethyl revealed resistance factors at ED50 and ED90 ranging respectively from 11 to 142, from 2 to 15 and from 4 to 6, thus confirming the prevalence of resistance to ALS-inhibitors in A. spica-venti. In greenhouse experiments, percentage canopy cover after herbicide treatment was determined in susceptible and resistant populations for the herbicides sulfosulfuron and fenoxaprop-P-ethyl by using digital image analysis. A significant effect of herbicide dose on canopy cover was observed in susceptible plants 7 and 15 days after treatment with sulfosulfuron, as well as in all populations when treated with fenoxaprop-P-ethyl. Canopy cover correlated significantly with plant dry weight in all populations, thus indicating that digital image analysis may represent a valid alternative approach to whole-plant bioassays and dose-response analysis for estimating biomass reduction after herbicide treatment. This work provides weed scientists with reliable tools for the verification of herbicide resistance in suspected weed populations.

doi: 10.1007/BF03356379pmid: N/A