Soil and plant root samples from a variety of native forest habitats throughout Australia were examined for potentially pathogenic Pythium and Phytophthora spp. by lupin baiting.Those It is suggested that disease of native plant species normally attributed to Ph. cinnamomi could be caused by other Pythium and Phytophthora species, acting singly or alone, with or without Ph. cinnamomi.
The soil-borne fungus Phytophthora cinnamomi Rands is a particularly impor-tant pathogen in Australia because of its consistent association with root-rot disease of a wide variety of exotic and native plant species. It was thought originally to have been introduced from south-east Asia (Crandall and Gravatt 1967), but evidence recently obtained (Pratt, Heather, and Shepherd, unpublished data), suggests that it may be indigenous to eastern Australia and may have been partly instrumental in determining the distribution of certain susceptible species, particularly Eucalyptu8 spp.
Determinations of cardinal temperatures for growth on various media of 50 Australian isolates of
Phytophthova cinnamomi showed that growth did not occur outside the range 5-35°C. The range
of temperatures at which growth optima occurred varied according to the isolate and medium used
and encompassed the whole range of values reported by overseas authors.
Growth rates of 361 isolates on corn meal agar at 25°C varied within the range 4.7-10.5 mm/day.
There was no correlation between optimum temperature and whether isolates were slow- or fastgrowing
or their place of origin. Fast-growing isolates (6-11 mm/day) were obtained from all States,
but slower-growing isolates (<6 mm/day) were obtained only from southern and western regions of
Australia. Populations from different regions of Australia exhibited different growth rate parameters.
The variability of mycelial isolates in culture was studied by examining differences in growth
rate among replicated parent, single-zoospore, single-zoosporangium and single terminal-hyphal
isolates. Extensive variation was found among first generation single-zoospore progenies of field
isolates, with lesser variation among progeny of single zoosporangia, terminal hyphal cultures and
second and third generation zoospore derivatives. The origin of this variation is discussed and it is
suggested that field isolates are heterokaryotic, since zoospores proved to be predominantly uninucleate.
When various Phytophthora species were incubated at temperatures above those at which growth
was possible and then returned to 25°C, their subsequent ability to resume growth depended on the
particular time-temperature combination used. Considerable variation of response was found among
a number of isolates of P. cinnamomi and, following the establishment of single zoospore isolates,
the potential variability of field isolates was shown to persist through successive generations of
zoospore propagation. It is suggested that a cytoplasmic mechanism of inheritance may be responsible
for this variation.
A total of 70 isolates of P. drechsleri from Australian native eucalypt forests were separated into two distinct ecotypes. The "northern" ecotype occurred from North Queensland to the south of New South Wales, while the "southern" ecotype occurred in South Australia, Victoria, Tasmania, and New South Wales.The northern ecotype on average grew faster on a variety of media than did the southern one, but ecotypes could not be recognized unequivocally on this basis. The upper temperature limit for growth of the northern ecotype was in the range 36-37' 5°C, while that for the southern was 33-36°C. Ecotypes could be separated on the bases of their growth rates in the presence of 33 .ug/ml Cu2+ ions (23' 54 and 55· 66 % of the growth in the absence of copper by the northern and southern ecotypes respectively), orin the presence of 1 p.p.m. pyronin G (63' 34 and 33· 87 % of the growth in the absence of pyronin G by the northern and southern ecotypes respectively). Both ecotypes showed similar pH optima for growth, similar responses to the effects of exposure at 44°C on subsequent growth at 25°C, and similar degrees of growth inhibition by 0·05 p.p.m. malachite green.It is tentatively suggested that the geographic distribution of the southern ecotype is related to the area enclosed by the 29·4 °C (85°F) isotherm of average daily maximum temperature during the period November to March annually.
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