The influence of temperature, inoculum level and race of Fusarium oxysporum f.sp. cubense on the disease reaction of banana cv. Cavendish

Cavendish banana plants were grown in soil infested with different levels of microconidia of races 1 and 4 of Fusarium oxyspomm Schlect. f. sp. cubense (E.F. Smith) Snyd. & Hans. The 4-week-old plants were incubated at 20¦ and 28¦C in temperature controlled cabinets subject to natural light fluctuations. Race 1, which does not normally infect Cavendish in the field, infected the plants at both temperatures, as did race 4. Results indicated that temperature was primarily affecting plant growth rather than influencing the aggressiveness of the pathogen. Inoculum levels were also found to influence disease severity with a minimum of 4.5 x l04 microconidia/g dry weight of soil required before macroscopic symptoms were observed. The suitability of month-old tissue culture derived plants for screening for resistance and the evolution of the Cavendish attacking race 4 strains are discussed in the light of these findings

Uromyces appendiculatus var. Crassitunicatus, var. nov. from macroptilium atropurpureum

Morphological studies on the rust fungi of Macroptilium, Phaseolus and Vigna show that three taxa can be distinguished. The Vigna rust, Uromyces vignae, is maintained as a species distinct from the bean rust, U. appendiculatus var. appendiculatus. A new variety, Uromyces appendiculatus (Pers.) Unger var. crassitunicatus J. Irwin, is described from Macroptilium atropurpureum. It differs from var. appendiculatus in having urediniospores with thicker side walls and more closely spaced echinulae. The teliospores are similar to those of var. appendiculatus, except that the side walls are thicker; the side walls are darker, with lineal rows of warts over the entire surface, and a conspicuous channel in the base; the hyaline apical thickening is thinner; and pedicels are more broadly attached

Stomata as an infection court for Phytophthora megasperma f. sp. medicaginis in chickpea and a histological study of infection

Stomata located beneath the cotyledons of 7-day-old chickpea seedlings were the preferential infection court for zoos pores of Phytophthora megasperma f. sp. medicaginis. Zoospores also accumulated preferentially in the root-hair zone. Penetration usually was intercellular between anticlinal epidermal cell walls, but intracellular penetration did occur via infection pegs produced from swollen germ-tube tips. Although no differences in prepenetration or penetration events were found between a field resistant (CPl56564} and a susceptible cuhivar (ICC6334} of chickpea, hyphal growth rate of the fungus was greater through wounded roots of the susceptible cultivar than in the resistant cultivar. There was no significant difference between hyphal growth rates through wounded epicotyl tissue of the resistant and susceptible cultivars. Stomata located at or below the soil line could act as an infection court for zoospores present in surface water, and cultivars that express resistance in the root tissue would appear susceptible if the resistance was not also expressed in the hypocotyl and the lower epicotyl. A soil-line infection court also could be involved in spreading the disease via zoospores present in surface water in the field

Glasshouse and field screening of chickpea cultivars for resistance to Phytophthora megasperma f. sp. medicaginis

The resistance expressed by a range of chickpea cultivars to phytophthora root rot in glasshouse and field trials was examined. Van der Plank's compound interest equation was the most suitable transformation for providing parameters to describe epidemic development for most of the cultivars when compared to the simple interest equation by plotting residuals and coefficients of determination. The area under the disease progress curve (AUDPC) was significantly correlated with the final disease incidence (FDI), which was also significantly correlated with the apparent infection rate (r), but r and AUDPC were not significantly correlated. the AUDPC proved to be selatively indiscriminate in this study for selecting resistant cultivars, since the value for the susceptible control cultivar was high, and many cultivars therefore had statistically lower AUDPC values. Several cultivars (i.e. CQ1591, ICC7568 and 232-4) were resistant on the basis of both the FDI and r parameters in the field trial and considered worthy of further investigation. When the parameters FDI or r were used to categorise the disease reactions of cultivars in the field, only half of the 30 cultivars assessed showed similar responses in both the glasshouse and field trials. The glasshouse assay used here does not allow the detection of all potentially useful forms of field resistance, and further work is needed in the development of a glasshouse assay which gives an accurate representation of field response to Phytophthora megasperma Drechs. f. sp. medicaginis Kuan and Erwin

Ribosomal DNA as a molecular taxonomic marker for the group species colletotrichum gloeosporioides

Isolates of the group species Colletotrichum gloeosporioides were assessed for restriction fragment length polymorphisms in their ribosomal DNA. Polymorphisms involving both length variations and a number of restriction enzyme sites were observed. There was sufficient variation amongst isolates to indicate that ribosomal DNA has potential as a taxonomic probe in this species

Development of a glasshouse assay for screening for partial resistance to P. vignae in cowpea

A glasshouse-based root inoculation method for screening for partial resistance to P. vignae was developed. By infecting potting mix with 0.01 g dry weight mycelium kg- dry weight of mix, inoculum levels comparable to those found under natural conditions of infestation in the field were obtained. This screening assay was used to compare resistance levels of eight cowpea lines with those obtained following hypocotyl inoculation with races 1-4 of P. vignae. One of the lines tested, CPI 84853, expressed partial resistance to all four races in the root inoculation assay, although it was highly susceptible to race 4 following hypocotyl inoculation. This line, along with the root assay described in this paper, form a basis for the development of new cowpea cultivars with more durable resistance to P. vignae

Glasshouse and field evaluation of cowpea lines for partial resistance to Phytophthora vignae

The reaction of cowpea lines to naturally occurring Phytophthora vignae in the held was studied in three trials over the period 1987-89 with the aim of identifying and characterizing lines with partial resistance. Seven lines (CPI 84853, CPI 84851, CPI 65393, CPI 77359, CPI 78209, CPI 78213 and cultivar Blackeye 5) were found to be partially resistant. Partial resistance was assessed by calculating disease incidence, disease severity, time lag before onset of disease, relative area under the disease progress curve and apparent mortality and infection rates. Significant differences in some of the characters further divided the partially resistant lines into different groups. It was concluded that the characters found to be most useful in the differentiation of relative levels of partial resistance were final percentage infection, and final percentage of plants with non-killing lesions. There were also significant correlations between these variables within and between years, although the relative rankings of some of the lines changed between years. Overall, CPI 84853 and CPI 65393 showed higher levels of partial resistance in the held than the remaining lines. In general, the partially resistant lines gave variable reactions following hypocotyl inoculation with P. vignae. There were clear indications of host-isolate interactions, suggesting that partial resistance may be no more durable than complete resistance for this particular host-pathogen interaction

Taxonomic studies on Australian isolates of stemphylium spp. and associated teleomorphs

Taxonomic studies on monosporic pure cultures of Australian isolates of Stemphylium revealed the presence of three species, namely S. botryosum, S. globuliferum and S. vesicarium. A group intermediate between S. botryosum and S. vesicarium but more closely resembling S. vesicarium was recognised. The circumscription of S. vesicarium was broadened to accommodate this taxon which was found on Medicago sativa, M. rugosa and Simmondsia chinensis. Studies on herbarium specimens deposited as S. botryosum on M. sativa from a number of overseas locations indicated that in all cases the pathogen fitted our concept of S. vesicarium. Teleomorphs were induced in pure culture for each of the above-mentioned anamorphs, and in all instances the teleomorph was identified as Pleospora herbarum. The ascospore morphology was similar for all isolates, and the shape of the ends of the spore and the number of longitudinal septa were variable characters even within an isolate. However, the teleomorphs could be separated into two distinct groups on the basis of ascal length

Vegetative compatibility and distribution of Fusarium oxysporum f.sp. cubense in Australia

Isolates of Fusarium oxysporum f. sp. cubense from wilted banana plants in Queensland and New South Wales were characterised for vegetative compatibility. Six vegetative compatibility groups VCGs) were identified. Race 1 (VCGs 0124, 01241.5, 0125) was widespread, being detected in northern and southern Queensland as well as northern New South Wales. Race 2 (VCG 0128) was found attacking Bluggoe in North Queensland. Race 4 (VCGs 0120, 0129, 01211) was detected in Cavendish plantations in southern Queensland and in Lady finger plantations in New (South Wales and southern Queensland. Isolates of the race 4 VCG 0129 from Lady finger plantations were pathogenic to Cavendish cultivars in glasshouse tests

Interaction of cowpea with Phytophthora vignae: inheritance of resistance and production of phenylalanine ammonia-lyase as a resistance response

The interactions between Phytophthora vignae and two cowpea cultivars are described. The cultivars are near-isogenic but differ in their resistance to P. vignae race 2. Cultivar Caloona is resistant, cv. Poona is susceptible. Resistance of cv. Caloona to race 2 is controlled by a single dominant gene and is expressed throughout the life of the plant in all tissues at temperatures up to 35°C. Inoculation of cut cowpea epicotyls with a plug of agar containing race 2 hyphae provides a reliable method of obtaining host tissue which responds in a predictable way to the pathogen. Phenylalanine ammonia-lyase (PAL) activity increases rapidly in invaded epicotyl tissue of cv. Caloona in response to inoculation with race 2. Increased PAL activity in advance of the hyphae is not detected. Low levels of PAL are produced by cv. Poona and cv. Caloona in response to inoculation with a race virulent on both cultivars (race 3). Application of l-2-aminooxy-3-phenylpropionic acid (1 mm) or aminooxy acetic acid (10 mm) through the cut bases of hypocotyls of cv. Caloona seedlings results in this cultivar, normally resistant to P. vignae race 2, becoming susceptible