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Stress prior to defoliation (70% open or 4 Nodes above Cracked Boll (NACB)) can cause a reduction in yield and fibre quality.
Good management of soil biodiversity is essential for efficient and productive farming. Manipulation of soil biodiversity may provide a means for managing soil borne pathogens in crop environments. Experiments described here failed to show that manipulation of microbial diversity in crop soil has a significant impact on black root rot in cotton plants.
Comparison of disease severity between cropped and uncropped soils showed that cotton plants grown in uncropped soils recovered faster from black root rot. It isn’t clear whether this was due to biological factors or abiotic factors. Plants grown in cropped soil were significantly smaller after 29 days than those grown in uncropped soil, although the size difference was significant only for shoot fresh weights and only apparent for other measurements, suggesting that faster recovery from black root rot allowed the plants to grow faster. However this pattern was mirrored when plants were not inoculated with T. basicola. Although the inoculated plants had less disease when grown in uncropped soil, all plants were larger when grown in uncropped soil indicating that the lower disease rate is more likely to be because of more rapid progression through the disease cycle. T. basicola infects the outer epidermis of cotton seedlings after about a week’s growth and causes significant blackening. After about five to six weeks, cotton plants in the field slough of the outer epidermal layer and recover from disease symptoms (Nehl, personal communication). The results of the soils comparison are inconclusive but suggest that there may be a relationship between soil biodiversity and disease severity. In order to fully understand the situation it is necessary to compare a larger number of soils. Also, whilst there is evidence that cropped soils are less diverse than uncropped or natural soils, it would be better to measure the biodiversity of all soils compared.
Reintroduction of selected fungi did not affect disease severity in cotton seedlings. This could be due to a number of factors: the selected microbes do not possess the ability to antagonise T. basicola; the inoculation method failed to set up suitable conditions for antagonism; or the density of T. basicola spores was sufficient to overcome the antagonistic action of the reintroduced microbes. Firstly, it has been shown that some soil saprotrophs have antagonistic action against T. basicola (Tashlieva, 1980) hence the limited range of microbes reintroduced may not be a representative sample. Secondly, an ecosystem is a complex and dynamically functioning entity which is not represented adequately by reintroduction of microbes into controlled conditions. Finally, T. basicola was reintroduced at a density much higher than observed field-levels; inoculation density may have overloaded the antagonistic potential of the artificial system. These considerations indicate that ecosystem complexity makes investigation of the impact of biodiversity on crop pathogenesis difficult.
Screening of eight soil saprophytes isolated from the Namoi Valley revealed that at least one fungus produces metabolites that are inhibitory to T, basicola. Although only a 17% inhibition of radial growth was observed it is possible that wider screening of microbes native to the region would reveal many more inhibitory microbes. It is possible that a microbe that produced strongly inhibitory metabolites could be found and utilised in management of the disease. Alternatively it is possible that a large number of weakly inhibitory microbes already exist in native soils, and that promoting these microbes in crop soils may help in the management of the pathogen.
The three experiments reported here indicate that there may be some truth to the idea that greater soil biodiversity could be useful in the management of crop pathogens.
