In pot and field experiments in naturally infested soil, commercial products
formulated with microbial antagonists, applied to soil, performed poorly
against the nematodes Tylenchulus semipenetrans and
Paratrichodorus lobatus, and the root rot fungi
Phytophthora nicotianae and
Pythium ultimum on citrus, compared with the
conventional chemicals aldicarb, cadusafos and metalaxyl.
Prosper Nema did not reduce nematode levels even when combined with a sawdust
mulch over the soil. Nutri-life 3/20 inhibited citrus growth and caused
leaf chlorosis. Tri-D25 did not reduce root rot or isolation frequency of
Phytophthora nicotianae and
Pythium ultimum. However, a commercial formulation
(Actizyme) of Bacillus subtilis stimulated citrus
growth, and the herbicide oryzalin both stimulated citrus growth and reduced
isolation frequency of Pythium ultimum. Potassium
silicate did not reduce isolation frequency of
Phytophthora nicotianae and
Pythium ultimum or root rot, however, it did reduce
levels of T. semipenetrans in soil. Cadusafos and
aldicarb were equally effective in controlling
T. semipenetrans on roots but cadusafos was more
effective in soil and against Paratrichodorus lobatus.
Aldicarb stimulated citrus growth, while cadusafos plus metalaxyl was more
effective than metalaxyl alone in stimulating citrus growth and preventing
leaf chlorosis in soil infected with
Phytophthora nicotianae.
Results suggested that Phytophthora nicotianae was much
more damaging to citrus than Pythium ultimum, and that
nematodes (in association with Fusarium solani) were
also more damaging than P. ultimum. Relatively low
initial combined levels of T. semipenetrans and
Paratrichodorus lobatus depressed citrus growth;
reproduction rates for P. lobatus exceeded 23-fold the
initial level. Cadusafos (3 applications at 3 g/m2
over 6 months) reduced T. semipenetrans to very low
levels, but did not eliminate the nematode; mechanical incorporation of
nematicide granules improved the level of nematode control after the first,
but not succeeding, applications.
Patches of stunted onion plants are common within otherwise healthy crops in the "Mallee" region of South Australia. A number of fungi including species of Rhizoctonia, Fusarium, Pyrenochaeta and Pythium were isolated from the roots of stunted plants, but only R. solani anastomosis groups (AG) 2.1 and 8 consistently caused severe stunting when onion seedlings were grown in soil inoculated with these pathogens. High quantities of R. solani AG 8 DNA were associated with soil collected from stunted patches but the pathogen was generally low or absent in soil from healthy areas. DNA of AG 2.1 and 3 were common in onion field soil, but quantities were not correlated with stunted areas. AGs 2.2, 4 and 5 were rarely detected in onion field soil. Nematodes such as Pratylenchus, Paratylenchus, Paratrichodorus and Tylenchorhynchus were extracted from soil and/or onion roots but numbers were generally low and were similar in stunted and healthy areas. Overall these results suggest that onion stunting in South Australia is primarily associated with R. solani AG 8 although interactions with other fungi and/or nematodes may also be involved.
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