A new method for the control of soilborne plant pathogens was tested for its efficacy in two field experiments during two years. Plots were amended with fresh broccoli or grass (3.4 to 4.0 kg fresh weight m(-2)) or left nonamended, and covered with an airtight plastic cover (0.135 mm thick) or left noncovered. In plots amended with broccoli or grass and covered with plastic sheeting, anaerobic and strongly reducing soil conditions developed quickly, as indicated by rapid depletion of oxygen and a decrease in redox potential values to as low as -200 mV. After 15 weeks, survival of Fusarium oxysporum f. sp. asparagi, Rhizoctonia solani, and Verticillium dahliae in inoculum samples buried 15 cm deep was strongly reduced in amended, covered plots in both experiments. The pathogens were not or hardly inactivated in amended, noncovered soil or nonamended, covered soil. The latter indicates that thermal inactivation due to increased soil temperatures under the plastic cover was not involved in pathogen inactivation. The results show the potential for this approach to control various soilborne pathogens and that it may serve as an alternative to chemical soil disinfestation for high-value crops under conditions where other alternatives, such as solarization or soil flooding, are not effective or not feasible.
Compost made from organic household and garden waste was used to substitute part of the peat in potting mixtures used for growing woody ornamental nursery stock. The effects of amendment with compost on the colonization of potting mixture by Rhizoctonia solani (AG1) were studied in greenhouse experiments. A bioassay was developed using cucumber as a sensitive herbaceous test plant as a substitute for woody ornamental cuttings. Pathogen growth in the potting mixture was estimated by measuring the distance over which damping-off of seedlings occurred. Compost from two commercial composting facilities suppressed growth of R. solani in potting mixtures with 20% of the product when the compost was fresh (directly after delivery) or long matured (after 5 to 7 months of additional curing). In contrast, short-matured compost (1 month of additional curing) from the same batches stimulated pathogen growth. In vitro mycelial growth of R. solani on mixtures with mature compost was inhibited by microbial antagonism. Compost-amended potting mixtures responded differentially to the addition of cellulose powder; the effect on suppressiveness depended on curing time and origin of the compost. In long-matured compost, suppressiveness to R. solani was associated with high population densities of cellulolytic and oligotrophic actinomycetes. The ratio of the population density of actinomycetes to that of other bacteria was around 200-fold higher in mature suppressive compost than in conducive compost.
Samples of heavily infested crop residues were incorporated in static compost heaps (2.5-4.6 m 3) of the Indore type. Temperature increased to 50-70 ~ within 6 days depending on the type of crop residues used and the location within the heap. The heat phase (~40 ~ lasted 2-3 weeks and was followed by a c. 5-months maturation phase (<40 ~ Among the 17 pathogens tested, only Olpidium brassicae and one of the four formae speciales of Fusarium oxysporum that were tested survived composting, but also their inoculum was greatly reduced.Survival during specific phases of composting was studied by incorporation and retrieval of samples at various stages of the process. F. oxysporurn f. sp. melonis was completely inactivated and O. brassicae and Plasmodiophora brassicae were almost completely inactivated during the short heat phase. The three pathogens survived the long-lasting maturation phase without loss of viability. Heat evolved during composting was found to be the most important factor involved with sanitation of crop residues. The possible involvement of fungitoxic conversion products and microbial antagonism is discussed.
A survey was made to identify the most important soilborne fungal pathogens of asparagus crops in the Netherlands. Ten plants were selected from each of five fields with a young (1-4 y) first planting, five fields with an old (6-13 y) first planting and five fields with a young replanting. The analysis included fungi present in the stem base and the roots of plants with symptoms of foot and root rot or showing growth decline without specific disease symptoms. Isolates of each species were tested for pathogenicity to asparagus on aseptically grown plantlets on Knop's agar. Symptoms were caused by Fusarium oxysporum, F. culmorum, Botrytis cinerea, Penicillium verrucosum var. cyclopium, Cylindrocarpon didymum, Phialophora malorum, Phoma terrestris and Acremonium strictum. F. oxysporum was by far the most common species and was isolated from 80% of the plants. Not all of its isolates were pathogenic to asparagus. Symptoms were caused by 67%, 78% and 93% of the isolates obtained from young first plantings, old first plantings and replantings, respectively. F. culmorum was isolated from 31% of the plants. Two other notorious pathogens of asparagus, F. moniliforme and F. proliferatum, did not occur in our samples.Species causing symptoms in the vitro test that were found on more than 5% of the plants were additionally tested for their pathogenicity in pot experiments. F. oxysporum f.sp. asparagi caused severe foot md root rot, significantly reduced root weights and killed most of the plants. F. culmorum caused lesions )n the stem base often resulting in death of the plant. P. terrestris, a fungus only once reported as a 9athogen of asparagus, caused an extensive root rot, mainly of secondary roots that became reddish. The :ungus was isolated in only a few samples and is not to be regarded as an important pathogen in Dutch sparagus crops. P. malorum caused many small brown lesions on the stem base and incidentally also n the upper part of small main roots. This is the first report of its pathogenicity to asparagus. The angus is one of the organisms inciting spear 'rust' and it reduced crop quality rather than crop yield. P. errucosum var. cyclopium and C. didymum did not cause symptoms in pot experiments. Because of its predominance on plants with foot and root rot and its high virulence, F. oxysporum sp. asparagi was considered to be the main soilborne pathogen of asparagus in the Netherlands. Ltroduction'paragus (Asparagus officinalis L.) is one of the tin vegetable crops grown in the southeastern part of the Netherlands. The asparagus produced is almost exclusively white asparagus. In this area, asparagus has been grown for several decades and many growers do not have fresh land available.
With respect to sensitivity to benomyl in vitro four categories of fungi were distinguished; taxonomically related fungi usually proved to belong to the same category. Within the form-class Deuteromycetes a correlation appeared to exist between sensitivity to this fungicide and morphogenesis of conidia. Phoma betae, in contrast to all other Phoma spp., was shownto be highly resistant to benomyl. This, however, may be expected on account of its perfect state.The data on the in vitro antifungal activity of benomyl conformed to those on the effectiveness of benomyl against plant diseases.
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