Silvertop in perennial grass seed production: effects of type and timing of biotic stresses

Abstract

Seed production of perennial grasses is an important industry in Canada, but many fields exhibit high proportions of sterile heads with characteristic symptoms in a condition known as silvertop. In a 3-yr field study, biotic stress treatments were applied to caged plots of Kentucky bluegrass (Poa pratensis) and meadow bromegrass (Bromus riparius) to assess their effects on silvertop incidence. Treatments were (1) control, (2) addition of grass-feeding insects, primarily grass plant bugs (Miridae), (3) inoculation with Fusarium poae spores, (4) both insects and spores, and (5) mechanical piercing of seed heads above the last node (in a single year). Additionally, cores of the two grass species were potted and removed from the field in spring in each of 2 years. After 2, 4, and 6 wk intervals in a cold room at 4 °C to stagger plant development stage, the five treatments were applied to caged plants of both grasses, which were maintained in a greenhouse until seed harvest. In both studies, the numbers of healthy seed heads, percentage of heads with silvertop, and seed weights were determined. In the field, stress treatments infrequently and inconsistently affected silvertop levels. Regression showed that the relationship between silvertop and seed yield, although significant, explained little of the variability in yield. Silvertop developed in all treatments in the greenhouse; treatments had no effect on silvertop levels, which were highest in plants treated at the R1 (spikelets visible) growth stage. This suggests that this stage of the two grass species is most vulnerable to silvertop occurrence.