Pathogenicity of Pythium spiculum and Pythium sterilum on feeder roots of Quercus rotundifolia

A severe decline affecting Quercus suber and Quercus rotundifolia has been reported since the early 1990s in southern Spain and Portugal. Root rot caused by Phytophthora cinnamomi has been the main factor involved in the decline (Brasier et al., 1996). Since 2003, Pythium spiculum (Paul et al., 2006) and Pythium sterilum (Belbahri et al., 2006) were isolated from feeder roots of declining oaks and from soil samples but there is no information about their pathogenicity on oak.

Pathogenicity tests were conducted on 4-year-old Q. rotundifolia plants inoculated with nine isolates of Py. spiculum, one isolate of Py. sterilum and one isolate of P. cinnamomi. Inocula were prepared by shaking and mixing the mycelium produced in 3 carrot broth Petri dishes per 100 ml of sterile water, before adding to each root ball to be inoculated. There were ten inoculated and non-inoculated control plants each per isolate. All the plants were waterlogged to favour root infection and maintained in a growth chamber (17-25ºC and 12 h light per day). One month later, the inoculated plants showed symptoms of root necrosis, that resulted in foliar wilting followed occasionally by defoliation. Control plants did not develop foliar symptoms and showed a very low level of root necrosis, averaging 0.6 on a 0-4 scale (0 = healthy root, 4 = dead root) used to assess root symptoms (Sánchez et al., 2005). Analysis of variance and comparison of means by the Fisher's LSD test was performed for root necrosis values. Root damage severity exhibited significant differences in relation to the control plants for all the isolates tested, with P. cinnamomi the most pathogenic isolate (average root necrosis value 2.8). Isolates PE154, PA053, PA054, PA055 ,PE137, PE138, PE144 of Py. spiculum produced values of root necrosis (2.2 in average) similar than isolate PE101 of Py. sterilum (averaging 2.1), and significantly higher than isolates PE155 and PE156 of Py. spiculum, averaging 1.6. The inoculated oomycete was always reisolated from necrotic roots and never from the roots of control plants.

This is the first report of Py. spiculum causing root rot on Quercus in Spain and Portugal. The root disease is similar in symptomatology to that caused by P. cinnamomi, with lower disease severities depending on the isolate. Pythium sterilum appears to be a weaker pathogen in comparison with P. cinnamomi, but does nether-the-less does have the potential to cause root disease on Quercus spp.