Joint action of disease control measures: a case study of alternaria leaf blight of carrot.

ABSTRACT The efficacy of chemical (i.e., foliar fungicide sprays), genetic (i.e., moderately resistant cultivars), and cultural (i.e., drip-irrigation system) control measures was quantified individually and in combination in the management of Alternaria dauci, the causal agent of Alternaria leaf blight of carrot. Whereas host resistance and drip irrigation affected both the time of disease onset and the rate of disease progression, chemical control affected only the latter. In all cases, a single control measure did not provide an acceptable level of disease suppression. Control efficacy values (based on the relative area under the disease progress curve) for chemical, genetic, and cultural control were 58 +/- 11, 39 +/- 20, and 60 +/- 22%, respectively (values are means +/- standard error). By contrast, implementing two control measures concurrently always improved disease suppression significantly compared with the individual measures. Control efficacy values were 91 +/- 8% for the integration of chemical and genetic measures and 82 +/- 23% for the integration of chemical and cultural measures. Moreover, yields in plots protected by two control measures simultaneously were higher by 10.1 to 28.6 t/ha than those in the respective plots protected by single measures. The joint effect of chemical control and host resistance was additive, whereas that of chemical control and drip irrigation was synergistic in most cases. A literature review was performed to determine if these findings represent a general relationship between chemical and genetic, and chemical and cultural measures. Based on 19 reviewed cases, it was concluded that additive effects are the rule and synergistic or antagonistic effects are the exception. Synergistic effects of two control measures were observed when one control measure improved the efficacy of the other directly or when one control measure induced host resistance or predisposed the pathogen to increased susceptibility. These results may enable a more effective selection of candidate control measures for integration in the future.

First Report of Leaf Blight Caused by Alternaria longipes on Carrots in Israel.

Alternaria leaf blight, caused by Alternaria dauci (Köhn) Groves & Skolko, is one of the most devastating foliar pathogens of carrots (Daucus carota L.). Lesions appear as minute, necrotic, dark brown spots often initiated on the edge of the leaflet blade. They later enlarge in size and may merge into a large necrotic area, causing shriveling of the entire leaflet (1). In summer 2000, observations made in several carrot fields in the northwestern part of the Negev Region in Israel revealed infections that were atypical for A. dauci because they were initiated primarily in the middle section of the leaflet blade and were surrounded by a large yellowish area. A. longipes (Ellis & Everh.) E. Mason was consistently isolated from the lesions. Occasionally both A. longipes and A. dauci developed on the same leaves. The two pathogens differed in conidial morphology (size and shape of spore and beak) when cultured on potato dextrose agar medium. One hundred conidia of each species were measured. A. dauci conidia were 100 to 450 µm long and 6 to 15 µm wide, with a beak of up to 3 times the length of the conidium; A. longipes conidia were 35 to 110 µm long and 11 to 21 µm wide, and the beak measured one-third to one-half the length of the conidium. These measurements corresponded to the sizes listed previously (2). Inoculation of greenhouse-grown plants and completion of Koch's postulates confirmed that A. longipes is pathogenic to carrots. Conidia of both species germinated at temperatures from 5 to 36°C. In vitro tests revealed that A. longipes was less sensitive than A. dauci to fungicides commonly used in Israel in carrot fields. A fifty percent effective dose of chlorothalonil and difenoconazole was 3.0 and 0.2 µg a.i./ml, respectively, for mycelia growth of A. dauci, whereas the corresponding values for A. longipes were 10.5 and 3.0 µg a.i./ml, respectively. The prevalence of A. longipes in carrot fields and the influence of this pathogen on yields are currently not known. References: (1) I. Barash et al. Physiol. Plant Pathol. 19:7, 1981. (2) M. B. Ellis. Dematiaceous Hyphomycetes. CMI. Kew, Surrey, England, 1971.

Optimization of Chemical Suppression of Alternaria dauci, the Causal Agent of Alternaria Leaf Blight in Carrots.

Alternaria leaf blight, caused by Alternaria dauci, is a major constraint to carrot production in Israel. Israeli carrot growers apply prophylactic sprays at 3- to 10-day intervals throughout the season until harvest, up to 30 sprays in a growing season. In this study, we attempted to optimize the chemical suppression of the disease, in order to reduce fungicide use. The efficacy of nine fungicides was determined in two field experiments. All fungicides reduced disease severity, but there were significant differences in efficacy among them. The most effective were difenoconazole and chlorothalonil; less effective were copper hydroxide, tebuconazole, trifloxystrobin, and mancozeb; the least effective in our experiments were flutrifol, propineb, and iprodione. The effect of the time of spray initiation on fungicide efficacy was determined in three field experiments. Qualitative (analysis of variance) and quantitative (regression) analyses of the data revealed that initiating sprays after disease onset reduced control efficacy. Thus, an action threshold model could not be developed for A. dauci in carrots. The time before harvest at which sprays could be terminated was tested in two field experiments and it was found that terminating sprays 14 days before harvest did not significantly affect the overall control efficacy. The main conclusions derived from these experiments were tested and corroborated in two additional field experiments.

Sporulation of Fusarium oxysporum f. sp. lycopersici on Stem Surfaces of Tomato Plants and Aerial Dissemination of Inoculum.

ABSTRACT Plants exhibiting symptoms of wilt and xylem discoloration typical of Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici were observed in greenhouses of cherry tomatoes at various sites in Israel. However, the lower stems of some of these plants were covered with a pink layer of macroconidia of F. oxysporum. This sign resembles the sporulating layer on stems of tomato plants infected with F. oxysporum f. sp. radicis-lycopersici, which causes the crown and root rot disease. Monoconidial isolates of F. oxysporum from diseased plants were assigned to vegetative compatibility group 0030 of F. oxysporum f. sp. lycopersici and identified as belonging to race 1 of F. oxysporum f. sp. lycopersici. The possibility of coinfection with F. oxysporum f. sp. lycopersici and F. oxysporum f. sp. radicis-lycopersici was excluded by testing several macroconidia from each plant. Airborne propagules of F. oxysporum f. sp. lycopersici were trapped on selective medium in greenhouses in which plants with a sporulating layer had been growing. Sporulation on stems was reproduced by inoculating tomato plants with races 1 and 2 of F. oxysporum f. sp. lycopersici. This phenomenon has not been reported previously with F. oxysporum f. sp. lycopersici and might be connected to specific environmental conditions, e.g., high humidity. The sporulation of F. oxysporum f. sp. lycopersici on plant stems and the resultant aerial dissemination of macroconidia may have serious epidemiological consequences. Sanitation of the greenhouse structure, as part of a holistic disease management approach, is necessary to ensure effective disease control.

Nematicide seed dressing for cyst and lesion nematode control in wheat.

A trial was conducted in the northern Negev region of Israel in a field heavily infested with both cyst (Heterodera avenae) and lesion (Pratylenchus mediterraneus) nematodes. Wheat (Triticum aestivum cv. Bet Lehem) seeds were coated with either seed-dressing formulation of furathiocarb 10 g a.i./kg seed or emulsifiable concentrate formulations ofcarbofuran 10 g a.i./kg seed or oxamyl 3.6 g a.i./kg seed. Untreated seeds served as control. The trial was arranged in a randomized block design with six replicates. Two months after germination, the population density of H. avenae in three treatments was 10-20% of that of the control and that of P. mediterraneus was 20-35%. The height of the wheat plants in the treated plots was 150% of the control. Toward ripening, ear counts in the treatments were 20-31% higher than in the control. Grain yield of the treatments was 38-48% higher than that of the control.