Seedling disease in an annual legume: consequences for seedling mortality,plant size, and population seed production

The effect of seed and seedling mortality on plant population dynamics depe nds on the degree to which the growth and reproduction of surviving individ uals can compensate for the deaths that occur. To explore this issue, we so wed seeds of the annual Kummerowia stipulacea at three densities in sunken pots in the field. which contained either field soil, microwaved field soil . or microwaved field soil augmented with oospores of three Pythium species . High sowing density reduced seedling establishment and seedling size, but these effects were independent of the soil treatment. In the oospore-augme nted soil, seed and seedling survival was low. The surviving plants were in itially smaller but, at maturity, average plant size was greatest in the oo spore-augmented soil. compared to the other treatments. Total population se ed production was unaffected by soil treatment. suggesting that the effect of disease was limited to the seedling stage, with surviving plants release d from intraspecific competition. To test the hypothesis that the surviving plants in the oospore-augmented soil were more disease-resistant, seeds fr om each of the sowing density-soil type treatments were sown in a growth ch amber inoculation study. No evidence for selection for resistance was found . A second inoculation experiment revealed that oospore inoculum reduced pl ant numbers and mass regardless of whether field or microwaved soil was use d, suggesting that results from the field experiment were nor dependent on the use of microwaved soil. The findings of this study indicate that the ec ological effects of disease on individual plants and on plant populations a re not necessarily equivalent.