Can arbuscular mycorrhizal fungi be used to control the undesirable grass Poa annua on golf courses?

1. Poa annua (annual meadow-grass or annual bluegrass) is the most problema tic weed of temperate zone golf putting greens. In the UK there are no chem icals approved for its control, although several herbicides and plant growt h regulators are available in the USA. Reducing P. annua levels in fine tur f would greatly reduce the heavy reliance on pesticides and water that curr ently exists. 2. This paper reports on an observational and a manipulative study in golf putting greens, aimed at determining whether arbuscular mycorrhizal (AM) fu ngi have any potential for the reduction of this weed in fine turf. 3. All 18 greens on three golf courses were sampled, and in two courses a n egative relation between AM fungi and P. annua abundance was found, upholdi ng previous results. In greens where AM fungi were relatively common (as me asured by root colonization), P. annua was rare, and vice versa. Furthermor e, when the fungi were common, abundance of the desirable turfgrass Agrosti s stolonifera was greater. 4. Two explanations are suggested for these relations, a competitive one, i n which AM fungi alter the balance of competition between the two grasses, and an antagonistic one, in which the fungi may directly reduce the growth of P. annua. 5. In a manipulative experiment, where mycorrhizal inoculum was added to a golf green, the colonization level of A. stolonifera roots was enhanced, as was the abundance of this grass. Furthermore, there was a suggestion that adding inoculum could decrease the abundance of P. annua. 6. AM fungi have the potential to be a much more environmentally sound meth od of P. annua control in sports turf than the currently used chemicals.