Khaled A. El-Tarabily

There is a substantial and increasing effort in industrial microbiology and biotechnology to develop microbial inoculants as a means to improve the sustainability and profitability of rural activities whilst increasing productivity. Inoculants are being developed for use as microbial biofertilizers, biocontrol agents for weed suppression, biopesticides and bioremediation agents. All of these require the addition of microorganisms to complex microbial communities. Plant Growth Promoting Rhizosphere organisms (PGPRs) are good examples of microbes that might have important roles in agriculture. PGPRs inhabit plant root rhizospheres and can affect plant growth directly by nutrient solubilisation (P, Nand K) and production of plant growth regulators, and indirectly by suppression of plant pathogens by competition, release of antibiotics or siderophores.

Cavity spot is a major soil-borne disease of carrots worldwide. It reduces the quality or carrots so that they become unmarketable resulting in substantial economic losses to the grower. Cavity spot disease of carrots is caused by different species of the genus Pythium. In WesternAustralia, P. coloratum and P. sulcatum are the causal agents of cavity spot disease with P. coloratum causing more severe cavity spot than P. sulcatum. The application of lime has been reported to reduce the incidence and severity of cavity spot disease in Western Australia. The objectives of this investigation were to study the effect of lime or gypsum on the development of cavity spot disease of carrots in soil artificially infested with P. coloratum and attempt to discriminate between the effects of pH and calcium on disease development.

One hundred and seventy Pythium isolates obtained from carrot cavity spot lesions from a field in Western Australia were found to belong to either P. coloratum or P. sulcatum. In laboratory pathogenicity tests on mature carrots, all isolates of P. coloratum produced large brownish-black water soaked and depressed lesions. In comparison, only a few isolates of P. sulcatum produced lesions, all of which were small. In glasshouse pathogenicity trials, P. coloratum produced substantial and numerous lesions at an inoculum density of 0.5% (weight of millet seed-based inoculum/weight of soil), whilst P. sulcatum produced only a few small lesions at inoculum densities of 0.8 and 1% and none at 0.5%. This is the first record of P. coloratum as a casual agent of cavity spot disease of carrots.

Application of lime to a carrot field soil significantly reduced the incidence of cavity spot disease on carrots. It also increased soil microbial activity and the number of colony forming units (cfu) of aerobic bacteria, fluorescent pseudomonads, Gram negative bacteria actinomycetes and significantly decreased the cfu of filamentous fungi and yeast. Liming also increased the cfu of nonstreptomycete actinomycetes. These were isolated using polyvalent Streptomyces phages and the dry heat technique to reduce the dominance of streptomycete actinomycetes on isolation plates. The species isolated included Actinoplanes, Micromonospora, Streptoverticillium, Nocardia, Rhodococcus, Microbiospora, Actinomadura, Dactylosporangium and Streptosporangium. The numbers of actinomycetes antagonistic to P. coloratum increased in soil amended with lime.

Three polyvalent Streptomyces phages were used to isolate four Micromonospora species (M. carbonacea, M. chalcea, M. purpureochromogenes and M. inositola) from rhizosphere soils from a rehabilitated bauxite mine in Western Australia. Streptomyces violascens was isolated using the selective dry heat technique. M. carbonacea, M. chalcea and M. purpureochromogenes were found to produce cellulases in vitro and cause lysis of Phytophthora cinnamomi hyphae. Glasshouse trials showed that a cellulase-producing M. carbonacea isolate used in conjunction with an antibiotic-producing isolate of S. violascens, had a synergistic effect on the suppression of Phytophthora root-rot, and significantly promoted the growth of Banksia grandis. This study illustrates the importance of combining potential biological control agents with different antagonistic abilities, to control soil-borne plant pathogens.

Glasshouse trials were conducted in order to examine the effect of lime or gypsum amendments on the development of cavity spot disease of carrots in a field soil artificially infested with Pythium coloratum. Each amendment was applied to the soil at 4000 or 8000 kg ha-1. Only the lime applications significantly reduced the incidence of cavity spot disease. In unamended soil, the calcium concentration of carrot roots was significantly less than that of carrots grown in amended soils. There were no significant differences in the calcium concentration of carrot roots grown in soils with lime or gypsum added in the presence or absence of the pathogen. Calcium did not appear to play a direct role in the reduction of cavity spot disease. Reduction of the incidence of cavity spot appeared to be related to the increase in soil pH associated with the application of lime.

Actinomycetes were isolated by the use of normal standard isolation techniques (streptomycetes), polyvalent Streptomyces phages and dry heat techniques (non-streptomycetes), respectively. In addition to the standard in vitro antagonism tests on agar, a novel technique was developed to screen isolates on the surface of intact mature carrots in the presence of Pythium coloratum a causal agent of cavity spot disease of carrots. From over two hundred actinomycete isolates only seven were found to be strongly antagonistic against P. coloratum. These were identified as Streptomyces janthinus, S. cinerochromogenes, Actinoplanes philippinensis, Streptoverticillium netropsis, Micromonospora carbonaceae, Actinomadura rubra, and Streptosporangium albidum. In glasshouse trials these actinomycetes significantly reduced the incidence of cavity spot disease in soil infected with P. coloratum.