The roots of sugarcane that exhibit a yield decline (YD) effect when grown in the Tully district, north Queensland, are commonly infected by vesicular-arbuscular mycorrhizal fungi (VAMF). Correlative evidence from elsewhere suggests that mechanisms, such as long-term monocropping and fertilizer application, could select for less beneficial VAMF isolates, which may contribute to a YD of the host plant. Consequently, the hypothesis that VAMF isolates, inadvertently selected through the sugarcane monocropping system, may contribute to the observed YD was proposed. Moreover, little published information was available on whether VAMF isolates influence sugarcane growth. A series of experiments was then initiated with two primary objectives: firstly, to quantify VAMF found in canegrowing soils of the Tully district, north Queensland, and secondly, to examine isolates of Glomus darum, sourced from a YD site, and determine if they assisted sugarcane in P uptake, and to determine the degree of responsiveness of sugarcane to these isolates for growth and P uptake.
A range of soils that differed in cropping history and in levels of extractable P was used to quantify the populations and population dynamics of VAMF in these soils. Using a "Most Probable Number" (MPN) bioassay with sugarcane as host plants, the density of VAMF propagules ranged from 15 propagules per 100 g in a rainforest soil to 146-186 propagules per 100 g in a nearby cane-growing soil. A further bioassay of three soils using com as the host plant showed that the clearing and burning of rainforest country followed by a fallow period led to populations as low as 3 propagules per 100 g, but that sugarcane monocropping led to a recovery in VAMF populations to 498 propagules per 100 g after 10 years and 172 propagules per 100 g after 18 years. There was insufficient evidence to suggest that soil P levels have altered either the density of propagules or the composition of the mycorrhizal community. A field survey near Tully, north Queensland, indicated that while root infection in sugarcane is generally restricted to below 25%, infections decline further with successive ratoons and with the onset of the wet season (February) in comparison to infection in plant-cane and during the dry season (June). Spore surveys were restricted in number due to a considerable proportion of spores unable to procure an infection point. For example, at least 39 and 68% of the spores extracted from soils that had grown sugarcane for 10 and 18 years respectively, were found to be dormant or non-viable. Many of the above studies included population determinations of selected pathogenic microorganisms, including nematodes and oomycetous fungi. Negligible populations of the root rot fungus, Pachymetra, were present in YD soil, whilst only minor populations of Pythium were found. Root lesion nematode (Pratylenchus zed) and stunt nematode ( Tylenchorhynchus sp.), among a number of nematode species observed from YD soil, were likely candidates for contributing to YD.
Despite a more extensive infection in roots of sugarcane, VAMF isolates taken from the rainforest soil were less able to sporulate in sugarcane and were more sensitive to P than those taken from the canegrowing soil (2 vs. 40 spores per 100 g). Plants grown in the cane-growing soil maintained VAMF root infection despite the addition o f P and Zn fertilizers (mean o f 35%), whereas plants grown in the rainforest soil had a reduction in root infection with fertilizer application (25 vs. 15%).
A number of mycorrhizal isolates derived originally from a long-term cane-growing soil was added at 4 spores g-1 to a cane-growing soil (acid-extractable P, 43 mg kg -1) with 0, 50 or 500 kg P ha-1 applied. Yield responses indicated the effectiveness of the available isolates was not variable. In the absence of P fertilizer only, mycorrhizal sugarcane plants produced a greater (mean of 14% more) dry matter yield than the non-mycorrhizal sugarcane; these plants also had a greater mean P concentration (0.23%) in the TVD leaf than the non-mycorrhizal plants (0.13%). Application of 50 kg P ha-1 reduced sporulation but not infection, while 500 kg P ha-1 eliminated both sporulation and infection. An additional study on a P-deficient cane-growing soil (acid-extractable P, 4 mg kg-1) to which spores (4 spores g-1) and P fertilizer (0, 3, 9, 27, 80 and 240 kg ha-1) were added demonstrated that sugarcane is less able to benefit from a mycorrhizal association than com or soybean. Apart from those that received 240 kg P ha-1, mycorrhizal and non-mycorrhizal sugarcane plants took up equivalent amounts of P per plant for each P rate, and produced plants with equivalent dry weight of tops. Mycorrhizal com plants took up more P (P<0.05 at 27, 80, 240 kg ha-1) and produced more dry matter yield (PO.05 at 0, 3, 9, 27, 80 kg ha-1) than non-mycorrhizal control plants. Similarly, mycorrhizal soybean plants took up more P (P<0.05 at 0, 27, 80, 240 kg ha-1) and produced more dry matter yield (PO.05 at 0, 27, 80 kg ha-1) than non-mycorrhizal plants. Uptake of P in plant tops was found to be more indicative of responsiveness than P concentration or dry matter yield. The addition of spores at densities of up to 1 spore g-1 was unable to procure significant yield advantages even for P-deficient plants, but that when at least 4 spores g-1 were added to a low-P soil with small fertilizer additions, production of tops was benefited by over 20%.
Evidence has been presented to support the conclusion that when P levels are sufficiently low to be growth-limiting, sugarcane growth is slightly promoted by cane-growing mycorrhizal fungi, but that this benefit declines, presumably due to a modest C-cost, as adequate P is applied to plants. Supportive evidence to suggest a mycorrhizal association is responsible, in part, for the spectacular yield decline effects, as welldocumented in the sugarcane industry, is lacking. Sugarcane is less dependent than corn and soybean on mycorrhizal associations for P uptake, even in an extremely low P soil. The responses in plant growth to mycorrhizal propagules from cane-growing soils are vastly different to those of plants that are grown in similar soils which remain in the uncleared state.