P. J. Keane

Leaf diseases of Eucalyptus associated with Mycosphaerella species

Two distinct types of leaf spot on juvenile leaves of Eucalyptus globulus were found to be caused by two species of Mycosphaerella. M. cryptica caused a small circular spot while M. nubilosa caused a larger, spreading lesion. M. parva Park & Keane was only found on older lesions caused by M. nubilosa, and appears to be saprophytic. M. cryptica was also shown to be the cause of large, blighting lesions on mature foliage of a wide range of species from both major subgenera of Eucalyptus. Observations on symptom development, sporulation, pathogenicity and leaf penetration for M. cryptica and M. nubilosa are presented. Preliminary results of field studies are also given.

The epidemiology and control of ascochyta blight in field peas: a review

Ascochyta blight is one of the most important diseases affecting field peas. The disease occurs in almost all pea-growing regions of the world and can cause significant crop losses when conditions are favourable for an epidemic. Here we review current knowledge of the epidemiology of the disease. Details are provided of disease symptoms, the disease cycle and the taxonomy of the causal fungi, Ascochyta pisi, Mycosphaerella pinodes and Phoma pinodella. The importance of seed-, soil- and air-borne inoculum is discussed along with the factors that influence survival of the causal fungi in soil, on seed or associated with pea trash. Many studies have been reviewed to establish how the fungi responsible for the disease survives from year to year, how the disease becomes established in new crops and the conditions that favour disease development. Evidence is provided that crop rotation, destruction of infected pea trash and chemical seed treatments can significantly reduce the amount of primary inoculum. Later sowing of crops has been shown to reduce the incidence and severity of disease. Fungicides have been used successfully to control the disease, although the cost of their application can significantly reduce the profitability of the crop. The best long-term strategy for effective disease control appears to be the development of ascochyta blight resistant pea varieties. Reports of physiological specialisation in ascochyta blight fungi are also documented. Despite extensive screening of germplasm, relatively few sources of resistance to ascochyta blight fungi have been found in Pisum sativum. However, the discovery of much better sources of resistance in closely related species and the development of advanced breeding methods offer new possibilities for developing useful resistance.

Further Mycosphaerella species associated with leaf diseases of Eucalyptus

Three new species of Mycosphaerella are described from diseased leaves of Eucalyptus growing in a species trial in south-eastern Australia: Mycosphaerella marksii sp. nov. was associated with circular to irregular, sometimes confluent lesions, bordered by a prominent red-brown margin on several species of Eucalyptus; M. grandis sp. nov. and M. aggregata sp. nov. were associated with lesions confined to the leaf margin, extending from the tip almost back to the petiole, on leaves of E. grandis. It is likely that M. grandis is a primary parasite and M. aggregata a secondary invader. The species are distinguished mainly by their ascospore morphology and germination. Discrepancies between spore dimensions recorded in this and a previous study are explained by differences in the stains used.

Three Mycosphaerella species from leaf diseases of Eucalyptus

Three species of Mycosphaerella were found to be associated with two distinct types of lesion on juvenile leaves of Eucalyptus globulus. M. cryptica caused smaller circular lesions, but on mature leaves of many other eucalypt species it caused large blighting lesions. M. nubilosa has been found only on large, blighting lesions on juvenile leaves of E. globulus and three other species in the eucalypt subgenus Symphyomyrtus. A saprophytic species, which is described here as Mycosphaerella parva sp.nov., was commonly found on lesions in association with M. nubilosa.

Reduction of sclerotial inoculum of Sclerotinia sclerotiorum with Coniothyrium minitans.

Abstract Aspects of the biology of C. minitans and its potential for control of S. sclerotiorum were investigated. Temperatures below 7°C resulted in comparatively slow rates of germination and infection of sclerotia by C. minitans. The optimum temperature for germination, growth, infection of sclerotia, and destructive parasitism by C. minitans was 20°C. The optimum relative humidity for germination, growth and infection by C. minitans was above 95%. Autumn inoculations with suspensions of conidia, pycnidia and mycelium of C. minitans in the field resulted in negligible numbers of sclerotia remaining viable after 1 month. With culture-grown sclerotia 2 months were required for a similar reduction of sclerotial viability. In the absence of C. minitans mulching had no significant effect on sclerotial viability. In the presence of C. minitans mulching did, however, influence the viability and infection by C. minitans of culture-grown sclerotia. Populations of field sclerotia also differed from culture-grown sclerotia in that they harboured an internal population of microorganisms, which included C. minitans, and had a lower level of viability at the commencement of the treatments. A winter application of C. minitans did not result in significant infection of sclerotia nor in a reduction in viability of sclerotia. This failure is believed to have resulted from low temperatures and dry conditions.

Botryosphaeria species from Eucalyptus in Australia are pleoanamorphic, producing dichomera synanamorphs in culture.

Species within the genus Botryosphaeria include some of the most widespread and important pathogens of woody plants, and have been the focus of numerous taxonomic studies in recent years. It is currently accepted that anamorphs of Botryosphaeria belong to two distinct genera, Fusicoccum and Diplodia. Species within the genus Fusicoccum commonly produce aseptate, hyaline conidia. In the present study, fungi were isolated from foliage and wood of Eucalyptus in native forests and plantations in Australia. Although these fungi produced Dichomera anamorphs in culture, they clustered within the Fusicoccum clade of Botryosphaeria based on their ITS sequence data. Four species, Botryosphaeria dothidea, B. parva, B. ribis and B. australis produced Dichomera conidia in culture. The Dichomera synanamorphs are described for these four species of Botryosphaeria. In addition, falling within the Fusicoccum clade of Botryosphaeria, two species were found to be distinct from previously described Botryosphaeria spp. based on their ITS sequences, but synonymous with D. versiformis and D. eucalypti. These observations are currently unique to isolates from host trees within the genus Eucalyptus in Australia, and the pleoanamorphic nature of these species is discussed.

Biological control of Sclerotinia sclerotiorum on aerial parts of plants by the hyperparasite Coniothyrium minitans

Hyperparasitism by Coniothyrium minitans on Sclerotinia sclerotiorum growing on aerial parts of bean plants was studied in the laboratory and field. Laboratory studies showed that C. minitans effectively controlled S. sclerotiorum-induced disease on detached leaves and microscopic observations indicated that hyperparasitism on plants is similar to that in dual culture. Germination and growth of C. minitans were restricted on the leaves and this was found to be partly associated with the presence of an inhibitor in the leaf surface wax. Spraying trials in the glasshouse and the field established that aerial applications of C. minitans did not control disease, even though parasitism of mycelium of S. sclerotiorum was demonstrated. It appeared that the effectiveness of C. minitans was reduced by inhibition of its germination and growth on leaves and its slower growth rate relative to S. sclerotiorum. However, the number of sclerotia produced on C. minitans-treated plants was reduced by 55% relative to controls, and these sclerotia formed stipes and apothecia less frequently than did sclerotia from control or Benlate-treated plants. Reduction in sclerotial numbers and carpogenic viability is believed to have occurred as a result of nutrient stress due to hyperparasitism.

Further Mycosphaerella species causing leaf diseases of Eucalyptus

Three previously described species of Mycosphaerella are discussed in relation to their original descriptions and type specimens. M. eucalypti does not appear to be a species of Mycosphaerella . However M. heimii is a distinct species but no Latin diagnosis was provided by the original author and the type specimen could not be located. M. molleriana is also distinct but it has not yet been collected from eucalypts in Australia. Three new species are described from diseased eucalypt leaves collected in southern Australia: M. delegatensis sp.nov., M. swartii sp.nov., and M. walkeri sp.nov. M. swartii and M. walkeri have identical teleomorphs, but distinct anamorphs, Hendersonia eucalyptorum and H. fraseri respectively, and have different host ranges.

Mycosphaerella diseases of juvenile foliage in a eucalypt species and provenance trial in Victoria, Australia

Summary The incidence of mycosphaerella leaf diseases and the severity of seven species of Mycosphaerella were assessed on the juvenile foliage of 13 eucalypt species and 35 provenances in a 2–3 year old trial in south-eastern Australia. Significant variation was observed between the 13 host species, with E. cypellocarpa and E. nitens recording the highest disease incidence and E. elata and E. oreades having no assessable disease. While wide variation was observed between provenances within species, variation was significant only between provenances of E. nitens. Seven species of Mycosphaerella were recorded on diseased leaves collected from the trial. M. cryptica and M. nubilosa were the most damaging, with M. cryptica being the most common and having the widest host range.

The impact of three species of Mycosphaerella newly recorded on Eucalyptus in Western Australia

Three species of Mycosphaerella parasitic on leaves of Eucalyptus spp. (M. ccryptica on E. diversicolo,: E. globulus, E. marginata and E. patens; M. marksii on E. globulus and E. botryoides; and M. suberosa on E. globulus) are repbrted from Western Australia for the first time. This is also the first record of M. suberosa from outside Brazil. Brief descriptions of the symptoms and impact of the three diseases are given and their potential importance to forestry in western Australia is discussed.