1998 Iowa Turfgrass Research Report
Growth of Agrostis palustris in Response to Adventitious Root Infection by Species of Acremonium
Clinton. F. Hodges and Douglas A. Campbell
The form genus Acremonium is recognized primarily for its endophytic associations with numerous grass species (11, 13, 14) that range from mutualistic to pathogenic (16). The economic consequences of endophyte-infected grasses are both positive and negative. Negative effects include Fescue toxicosis and ryegrass staggers in livestock (2,6). Positive effects include increased insect, nematode, and pathogen tolerance (3, 11, 13, 14). Other positive effects include enhanced seed germination, growth, and drought resistance (1, 4, 5). Naturally occurring endophytes have not been reported in association with Agrostis palustris, but inoculations of A. palustris with a group of Acremonium isolates suggest that stolons can be infected (15). In recent years, research has focused primarily on the potential benefits afforded turfgrasses infected by Acremoniun endophytes.
Over the last 10 to 12 years isolation of Acremonium rutilum and A. alternatum from the roots and leaves of diseased A. palustris growing on high-sand-content golf greens has increased. Symptoms are expressed differently depending on whether the plants are on the closely mowed greens or on the collar of the green at higher mowing levels. Closely mowed greens show irregular areas of unthrifty, thinning turf due to mild undercover chlorosis and tanning of the older leaves. The higher mowed grasses of the collars show distinct brown, irregular patches of turf with living plants interspersed through the patches. The affected areas in the collar enlarge very slowly during the growing season and seem to be perennial in that they recur in the same location from season to season. Isolation of Acremonium species from A. palustris with these symptoms is most common during cool, wet periods of the spring and fall. The expression of symptoms, however, is most prevalent in the warmer periods of the summer.
Inoculation of adventitious roots of A. palustris with three isolates of A. rutilum and one isolate of A. alternatum under high (95° day/75° night) and low (75° day/60° night) temperature regimes decreased the growth of the plants, but no plants were killed. Under high temperatures, the various isolates of A. rutilum examined decreased shoot growth 13 to 58%; root growth was decreased 23 to 47%. A. alternatum induced a 68% decreased in shoot growth and a 56% decrease in root growth under the high temperatures. Under low temperatures, the isolates of A. rutilum decreased shoot growth 60 to 69% and root growth 51 to 62%. A. alternatum decreased shoot growth by 71% and root growth by 58% under the low temperatures. The observations show that growth of plants infected by A. rutilum is inhibited more under low temperatures than under high temperatures, and that the negative effect on growth by A. alternatum is temperature neutral.
The number of stolons produced by A. palustris infected by the Acremonium species decreased in response to high and low temperatures. Under high temperatures, stolon numbers per plant were reduced 21 to 35% in response to the isolates of A. rutilum and 36% in response to A. alternatum. The decrease in stolon numbers was more severe under the low temperatures; isolates of A. rutilum decreased stolon numbers in a range of 48 to 54% and A. alternatum decreased stolons by 60%.
Inoculated roots from both the high and low temperature regimes were typically tan colored without any evidence of lesions or rot and no visible signs of hyphae or spores. Vascular cylinders of some inoculated roots were cream colored, and some root tips were swollen. Some roots that were cleared and stained showed fine hyphal growth on and within the cortex from the low temperature study; visible hyphae on the surface of roots from the high temperature regime were rare. Among plants subjected to the high temperature regime, reisolation of the isolates of A. rutilum from roots ranged from 63 to 77%; A. alternatum was reisolated from 81% of the inoculated roots. Reisolation of A. rutilum isolates from inoculated roots of plants subjected to the low temperature regime ranged from 73 to 83%; A. alternatum was reisolated from 73% of the inoculated roots.
Decreases in shoot and root growth in response to isolates of A. rutilum and A. alternatum and their reisolation from inoculated roots establishes a degree of pathogenicity. Inoculated roots, however, showed only slight tanning, some yellowing of the vascular cylinder and scant hyphal development in the cortex. Also, reisolation of the Acremonium isolates from inoculated roots ranged only from 63 to 83%. These characteristics coupled with the inability of the Acremonium isolates to induce shoot symptoms suggest that they do not function as strong primary pathogens.
The developmental characteristics of A. rutilum and A. alternatum on roots of A. palustris suggest that these species may be emerging pathogens and/or constituents of root disease complexes. The modern high-sand-content green and the management practices applied to it are especially stressful and may predispose A. palustris roots to infection by weak pathogens and/or minor root pathogens (12). Evidence is growing that close moving and abrasion of roots by sand particles are predisposing roots to infection by relatively weak pathogens like A. rutilum and A. aternatum. Other weak and minor root pathogens showing similar characteristics on A. palustris roots include Idriella bolleyi, Curvularia lunata, and various species of Pythium (7, 8, 9, 10). All of these organisms, including the Acremonium species, are commonly isolated from the adventitious roots of the same plant. Each organism alone can induce some symptoms when they infect adventitious roots, but individually they cannot reproduce the field symptoms. These responses suggest that organisms like A. rutilum, A. alternatum, I. bolleyi, C. lunata, and some Pythium species are responding to the stressful conditions applied to A. palustris as a root-disease complex. It is also possible that the stressful management conditions are contributing to the emergence of pathogens from organisms that are typically benign under less stressful growing conditions.
The research presented was funded in part by grants from the Iowa Turfgrass Institute and the Iowa Golf Course Superintendents Association.
16. White, J. F., Jr. 1988. Endophyte-host associations in forage grasses. XI. A proposal concerning the origin and evolution. Mycologia 80: 442-446.
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ISU Horticulture:Publications:1998 Turfgrass Report | College of Agriculture |