John Hockenhull

Selection of biological control agents for controlling soil and seed-borne diseases in the field

Different screening methods for selection of biological control agents (BCAs), for controlling soil and seed-borne diseases, are discussed. The shortcomings of laboratory methods focused on mechanism of action are discussed and we conclude that these methods should be used with caution if candidates with multifactorial or plant mediated mechanisms of control are to be obtained. In vitro screens may be useful for specific groups of microorganisms, thus, screens for antibiotics may be relevant for Streptomyces spp., and promising results have been obtained using soil plating or precolonized agar methods to screen for mycoparasitism and competitive saprophytic ability. Experience with screening in the Nordic programme ‘Biological control of seed borne diseases in cereals’ is summarized. Research in the four participating countries – Finland, Sweden, Norway and Denmark – followed the same paradigm: that of obtaining antagonists, well adapted to different Nordic environments, and developing them as effective BCAs. Potential antagonists were isolated from different sources and in planta screening methods were developed in order to optimize selection of antagonists effective against a range of seed borne pathogens. Screens in the laboratory or greenhouse were followed by screening in the field. The different screening procedures are compared and evaluated.

Suppressiveness of organically and conventionally managed soils towards brown foot rot of barley

Abstract Five sandy loam soils under organic, integrated and conventional management were chosen to investigate the effect of specific agricultural management practices on suppression of brown foot rot of cereals caused by Fusarium culmorum. The relationships between suppressiveness and C and N content of the soil microbial biomass and microbial activity were investigated. Fungistasis tests and plant bioassays were compared. Differences in suppressiveness were most marked in plant bioassays following seed inoculation with F. culmorum. When inoculum was applied to soil as conidia or mycelia, soil-suppressed transmission of the pathogen and disease development. This finding indicates that the competitive saprophytic ability of the isolate of F. culmorum is poor. The microbial biomass and activity of the soils under organic and integrated farming were high. However, high biomass and activity were not always correlated with high disease suppression. Specific organic amendments, such as mulching with straw and the practice of using lucerne as a break-crop in cereal cultivation may influence inoculum potential of F. culmorum, disease outbreak and suppression. The possible significance of soil abiotic factors on disease suppression is discussed.

Biocontrol of Pythium tracheiphilum in Chinese Cabbage by Clonostachys rosea under Field Conditions

Control of leaf and head rot of Chinese cabbage (Brassica campestris L. ssp. pekinensis), caused by Pythium tracheiphilum, was obtained by Clonostachys rosea (isolate IK726) in field trials conducted in 1995 and 1999 on naturally infested land in a commercial crop in Denmark. A significant 2-3-fold disease reduction was obtained at an application rate of 108-109 conidia m−2 (high application rate) in both years, but not at a 10-fold reduced rate in 1999. Disease reduction by Trichoderma harzianum (Supresivit) was almost significant at the high application rate (1 g product m−2 corresponding to 7×109 colony forming units (CFUs) m−2) in both years, but not at a 10-fold reduced rate applied in 1999. In both 1995 and 1999 trials, the percentage of marketable heads increased significantly by 10% following a full application rate of C. rosea. Supresivit applied at the full application rate gave a significant 13% yield improvement in 1995 but not in 1999. No yield improvement was found when the two agents were applied at 10-fold reduced rates. A Danish T. harzianum isolate significantly increased yield by 13% in 1995, but gave no disease control. Plant growth promotion may have been responsible for yield improvements obtained by Supresivit and the Danish isolate of T. harzianum. The 1995 trial also evaluated the products Binab T (T. harzianum+T. polysporum), Mycostop (Streptomyces griseoviridis), Polyversum (P. oligandrum) and Aliette (fosetyl-Al) and Danish isolates of P. oligandrum (2) and T. virens (1), none of which gave disease control or yield effects.

Development of a biocontrol agent for plant disease control with special emphasis on the near commercial fungal antagonist Clonostachys rosea strain ‘IK726’

Numerous experiments demonstrating potential biocontrol effects on soilborne diseases have been reported in the scientific literature. However, from the lists of approved and registered biocontrol agents, it is striking how few have been commercialised and are used in practise for plant disease control. The main hindrances are often claimed to be legislative aspects and the costs involved in the registration. Although this is in many respects true, there is a range of both biological and technical problems which must be considered when developing an effective biocontrol agent for commercial use.Among the success stories for control of seed- and soilborne diseases are fungal biocontrol agents based on Trichoderma harzianum, Clonostachys rosea and Conithyrium minitans, and bacterial biocontrol agents based on strains of Agrobacterium, Pseudomonas and Streptomyces. We have developed C. rosea strain ‘IK726’, which has proved to be an effective antagonist in several crops against seed- and soilborne diseases. Although a biocontrol agent based on C. rosea ‘IK726’ is not yet commercialised, this paper will be used to address some of the biological and technical aspects that must be dealt with in such a development.

Distribution of Saprophytic Fungi Antagonistic to Fusarium Culmorum in Two Differently Cultivated Field Soils, with Special Emphasis on the Genus Fusarium

ABSTRACT The purposes of the present study were to search for antagonistic fungi to control seed-borne diseases of cereals caused by Fusarium culmorum (W.G.Sm) Sacc. and to use these results to compare the distribution of non-pathogenic Fusarium spp. and other fungi antagonistic to Fusarium culmorum in soil and on straw particles in an organically and a conventionally cultivated field. The organic farm had been cultivated according to the principles of Steiner (1963) since 1952. The sites were similar with respect to climatic conditions, soil type and vegetation. There was no difference in the total number of antagonistic fungi isolated from the two differently cultivated fields, but the results for Fusarium spp. showed effects of the actual crop as well as the cropping system. Soil sampling over a three year period at the organically cultivated farm in two crops—winter wheat or a mixture of grass and clover—showed almost twice as many species of Fusarium in the mixed crop compared to monoculture. In mixe...

Comparative studies on the effects of a yucca extract and acibenzolar-S-methyl (ASM) on inhibition of Venturia inaequalis in apple leaves

The effect of an extract of Yucca schidigera on the control and infection process of the apple scab pathogen, Venturia inaequalis, was examined and compared with the chemical resistance inducer, acibenzolar-S-methyl (ASM). In seedling assays, both materials significantly reduced apple scab symptoms and pathogen sporulation on leaves and both showed similar control efficacies as the reference treatment, sulphur. Whereas yucca extract and sulphur gave significant inhibition of conidial germination in vitro, ASM did not inhibit germination. Histopathological studies of the infection process of V. inaequalis in apple leaves showed that the yucca extract primarily acted by inhibiting pre-penetration events and penetration itself. In contrast, the ASM treatment significantly inhibited more stages of the infection process (pre-penetration, penetration and post-penetration events). These observations suggest that the yucca extract acted mainly by a direct fungitoxic effect whereas ASM, as expected, acted as a resistance inducer. However, expression studies of two genes encoding the PR proteins, PR1 and PR8, in apple seedlings indicated that yucca extract may also affect plant defence as expression of both genes was up-regulated following yucca treatment, to a level similar to that observed after treatment with ASM. The fungitoxic effect of sulphur on V. inaequalis was also confirmed in this study.

Potential suppressiveness of different field soils to Pythium damping-off of sugar beet

A highly reproducible, low resource requiring disease severity bioassay for testing suppression of Pythium damping-off in sugar beet caused by Pythium ultimum in soil is described. Changing of soil characteristics and the native microflora are avoided and standardization of the assay is carried out by testing infested soils on evenly germinating plants growing in an inert medium. The disease suppressiveness of the test soils is compared to standard, conducive soil, and quantified by calculation of symptoms on the hypocotyls. The reliability of the assay was investigated in nine well characterised organically or conventionally managed soils. In the standard, conducive soil it was clearly demonstrated that disease expression is correlated with available nutrient supply (oatmeal). Addition of oospores to the test soil had no effect on disease severity and the role of inoculum level is discussed. There was a clear tendency for low clay content to result in higher disease suppression. Partial sterilisation of the selected soils by γ-irradiation and steaming at 54 °C indicated that suppression was of microbial origin. Although texture may play a role in suppressiveness no clear effect, related to any management practice, was found. Tests on 64 soils confirmed that sandy soils tend to be more suppressive than clayey soils.

Field evaluation for resistance to the black rot pathogen Xanthomonas campestris pv. campestris in cabbage (Brassica oleracea)

Black rot, caused by Xanthomonas campestris pv. campestris, (Xcc), is one of the most serious diseases of crucifers world-wide. Forty-nine genotypes were evaluated for resistance under field conditions in Tanzania after artificial inoculation with Xcc race 1. Open pollinated white cabbage cultivars were generally susceptible, while Portuguese and pointed cabbages exhibited partial resistance. Some F1 white cabbage cultivars were highly susceptible, whereas others exhibited a high level of partial resistance. The most promising of the hybrid cultivars were T-689 F1, Gianty F1, No. 9690 F1, N 66 F1, and SWR-02 F1. Breeding line Badger I-16 exhibited the highest level of resistance of all genotypes. The genotypes accounted for 72.9–75.5% of the variation of the disease severity when assessed on the leaves, and 71.4% of the variation when assessed as internal black rot in heads at harvest. High correlations (equal to or above 0.7) were found between disease severities assessed on leaves three times during the growing season and also with the amount of internal black rot in heads. Leaf loss also was correlated with disease severity. The high genetic determination of the trait and the high correlations between disease assessments indicate that selection for resistance to black rot will be efficient when field screenings are carried out. Evaluation of genotypes for disease severity on leaves during the growing season combined with evaluations of head resistance in the most promising genotypes may be a simple method to select resistant cultivars.

Occurrence of Gliocladium Roseum on Barley Roots in Sand and Field Soil

The rhizosphere competence on barley of the antagonist, Gliocladium roseum (isolate IK 726), was investigated in semi-sterile sand and field soil. Following application of the antagonist to the seeds it was shown by the dilution plating method, that G. roseum was present on the roots. In sand colonization occurred most on proximal compared to distal parts of the roots. In field soil, 4 month after sowing, G. roseum was recovered from roots of plants derived both from seed inoculated with G. roseum and non inoculated with the antagonist. However, the population of G. roseum was significantly higher on roots derived from inoculated seeds.

The Use of Pythium Periplocum to Control Damping-Off of Cucumber Seedlings Caused by Pythium Aphanidermatum

Pythium periplocum Drechsler is a necrotrophic mycoparasite that has been recorded infrequently (Plaats-Niterink, 1986) and little studied. There is some evidence that it has a different distribution than-and, thus, probably differs physiologically and ecologically from- the better known mycoparasite, P. oligandrum Drechsler (Hendrix and Campbell, 1970). Some aspects of the use of P. periplocum as biological control agent are presented in connection with experiments aimed at the control of damping-off of cucumber seedlings caused by P. aphanidermatum (Edson) Fitzp.