Biological control of cucumber powdery mildew by Tilletiopsis minor

Ampelomyces strains isolated from diverse powdery mildew hosts in Japan: Their phylogeny and mycoparasitic activity, including timing and quantifying mycoparasitism of Pseudoidium neolycopersici on tomato

A total of 26 Ampelomyces strains were isolated from mycelia of six different powdery mildew species that naturally infected their host plants in Japan. These were characterized based on morphological characteristics and sequences of ribosomal DNA internal transcribed spacer (rDNA-ITS) regions and actin gene (ACT) fragments. Collected strains represented six different genotypes and were accommodated in three different clades of the genus Ampelomyces. Morphology of the strains agreed with that of other Ampelomyces strains, but none of the examined characters were associated with any groups identified in the genetic analysis. Five powdery mildew species were inoculated with eight selected Ampelomyces strains to study their mycoparasitic activity. In the inoculation experiments, all Ampelomyces strains successfully infected all tested powdery mildew species, and showed no significant differences in their mycoparasitic activity as determined by the number of Ampelomyces pycnidia developed in powdery mildew colonies. The mycoparasitic interaction between the eight selected Ampelomyces strains and the tomato powdery mildew fungus (Pseudoidium neolycopersici strain KTP-03) was studied experimentally in the laboratory using digital microscopic technologies. It was documented that the spores of the mycoparasites germinated on tomato leaves and their hyphae penetrated the hyphae of Ps. neolycopersici. Ampelomyces hyphae continued their growth internally, which initiated the atrophy of the powdery mildew conidiophores 5 days post inoculation (dpi); caused atrophy 6 dpi; and complete collapse of the parasitized conidiphores 7 dpi. Ampelomyces strains produced new intracellular pycnidia in Ps. neolycopersici conidiophores ca. 8-10 dpi, when Ps. neolycopersici hyphae were successfully destroyed by the mycoparasitic strain. Mature pycnidia released spores ca. 10-14 dpi, which became the sources of subsequent infections of the intact powdery mildew hyphae. Mature pycnidia contained each ca. 200 to 1,500 spores depending on the mycohost species and Ampelomyces strain. This is the first detailed analysis of Ampelomyces strains isolated in Japan, and the first timing and quantification of mycoparasitism of Ps. neolycopersici on tomato by phylogenetically diverse Ampelomyces strains using digital microscopic technologies. The developed model system is useful for future biocontrol and ecological studies on Ampelomyces mycoparasites.

Extensive colonization of apples by smut anamorphs causes a new postharvest disorder

Colonization of apples by ballistoconidium-forming fungi causes a new disorder, here named 'white haze'. White haze may occur in mild form in the field, but only becomes problematic after Ultra-Low Oxygen storage, and, therefore, may be considered as a postharvest disorder. All isolates, obtained using the spore-fall method, were morphologically identified as anamorphs of smut fungi belonging to the genus Tilletiopsis. Sequence analysis of the D1/D2 and the ITS domains of the rDNA revealed nine novel taxa scattered among the Exobasidiomycetidae (Ustilaginomycetes). Field experiments confirmed the erratic incidence of white haze over the years, and the development of the disorder seems to be enhanced at lower temperatures and a high relative humidity. Several scab fungicide treatments showed diminishing effects on the incidence of white haze.

Hydrolytic enzymes and antifungal compounds produced by Tilletiopsis species, phyllosphere yeasts that are antagonists of powdery mildew fungi

Isolates of five species of the yeast-like fungus Tilletiopsis Derx (Tilletiopsis albescens Gokhale, Tilletiopsis fulvescens Gokhale, Tilletiopsis minor Nyland, Tilletiopsis pallescens Gokhale, and Tilletiopsis washingtonensis Nyland) were screened for exo- and endo--beta-1,3-glucanase and chitinase production in a liquid broth used to produce inoculum for biological control studies. There were significant differences among the species, and highest overall enzyme activity was present in T albescens and T. pallescens and lowest in T. washingtonensis. A time-course study of beta-1,3-glucanase and chitinase production in T pallescens ATCC 96155 in broth culture with 2.5% glucose as the carbon source showed that enzyme activity gradually increased over a 3- to 21-day period. Maximum enzyme activity was found between pH 4.0 and 5.0. SDS-PAGE of beta-1,3-glucanase isozymes revealed a range of molecular masses from 18 to 29 kDa. Five isozymes were present in both T albescens and T. pallescens and two in T washingtonensis. Antifungal compounds were also detected in ethyl acetate extracts of culture filtrates of T. pallescens ATCC 96155 after 6 days of incubation, while no activity was detected at 14 days. One active fraction was selected following fractionation and preparative chromatography and was bioassayed against Podosphaera (sect. Sphaerotheca) xanthii (Castagne) U. Braun & N. Shishkoff and a number of other fungi. A concentration of 130 microg/mL inhibited germ tube development in P. xanthii, and mildew spores appeared plasmolyzed. Other fungi were inhibited at higher concentrations. Collapse of hyphae and conidiophores was also observed on mildewed leaves treated with the active fraction. Proton NMR analysis indicated that the inhibitory compound was a fatty acid ester. In 3- to 6-day-old cultures of T pallescens ATCC 96155 demonstrating biological control activity, antifungal compound production may have a primary role in restricting growth of mildew fungi and other competitors when applied to leaves.

Biological control of air-borne pathogens

Some pathogens are partly controlled by microorganisms that occur naturally on aerial surfaces of plants, and many attempts have been made to improve control by applying selected antagonists to such surfaces. Antagonists often compete for nutrients with the pathogen, and antibiotics may be formed that reduce germination of its spores and subsequent growth. Hyphae of fungal pathogens may be killed on contact with the antagonist or by direct penetration. The plant’s defences may be stimulated before challenge by a pathogen. Apart from killing the pathogen, an antagonist may reduce its reproductive capacity. The examples given illustrate the operation of these different mechanisms in the control of a wide variety of diseases. For diseases of foliage, flowers or fruit, glasshouse crops offer more attractive possibilities for control than field crops because the population level of antagonists is easier to maintain. In some cases plants can be protected by inoculation before transplanting them to the field. Foliage and canker diseases of forest trees present problems too intractable for successful control, but in orchards the prospects are better; for example, methods are available for combining pruning with application of inoculum. Similarly, in some circumstances tree stumps can be inoculated to prevent colonization by a pathogen. Where biological methods are as effective as chemical ones and comparable in cost, they are to be preferred on environmental grounds. In some cases they can be combined with advantage; for example a lower concentration of fungicide may suffice if applied with an antagonist.