Infectivity and Sporulation of Phytophthora ramorum on Northern Red Oak and Chestnut Oak

We evaluated the effect of moisture period on foliar disease development by Phytophthora ramorum on 2- to 3-year-old northern red oak (Quercus rubra) and chestnut oak (Q. prinus). We also determined the propensity of P. ramorum to form sporangia and chlamydospores on these two host species. Leaves were dip-inoculated with ca. 5000 sporangia/ml of P. ramorum isolate Pr-6 and incubated at 100% relative humidity in dew chambers in darkness for up to 6 days. Several plants were removed each day to a greenhouse, and foliar infection was evaluated on day 7. Sporangia were collected over a 7-day period from diseased foliage in a mist tent. A significant relationship between moisture period and disease incidence was found for both tree species. Chestnut oak exhibited significantly greater disease incidence and severity compared with northern red oak. However, sporulation levels were larger in northern red oak over the 7-day period of sporangia collection, and northern red oak also produced significantly greater numbers of sporangia per square centimetre of lesion area compared with chestnut oak. Chlamydospore production in diseased leaves sampled 1 month following moist incubation was also significantly greater for northern red oak compared with chestnut oak. Knowledge of P. ramorum sporulation capacity in relation to disease incidence and severity on Eastern US oak species will help determine the potential for epidemic development should the pathogen become established in this region.

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