Root infection by Phytophthora cinnamomi in seedlings of three oak species

Abstract

The combination of soil infestation with Phytophthora cinnamomi and repetitive flooding was studied on 1-year-old plants of Quercus ilex (holm oak) and Q. suber (cork oak). In a second experiment, using 2-year-old plants of the same species and of red oak (Q. rubra), the soil infestation was followed by two drought-rewatering cycles. Oak predawn leaf water potential (PLWP) and stomatal conductance (gs) were monitored during both experiments. Root infection, root loss, wilting and mortality were assessed at the end of the experiments. Q. ilex exhibited the highest susceptibility to P. cinnamomi, and Q. rubra the lowest. Root infections caused by P. cinnamomi were more severe in the flooding than in the drought experiment. The most noticeable effect of the infection on plant water relations was a decrease in stomatal conductance. This occurred at different times after inoculation, varying with species susceptibility and experiment. Inoculation with P. cinnamomi induced a decrease of PLWP in Q. ilex plants, and in some Q. suber plants exhibiting a severe root loss. The results further showed that the relationship between PLWP and gs was modified by infection with P. cinnamomi. The combination of flooding and infection with P. cinnamomi acted synergistically on the water relations of Q. ilex. By contrast, there was no significant increase in disease severity due to the postinoculation water stress imposed on the oaks.

Differences in root growth of <i>Quercus ilex</i> and <i>Quercus suber</i> seedlings infected with <i>Phytophthora cinnamomi</i>

“…Notably, however, regarding the symptoms observed in the roots, we found differences between the species. In the Q. ilex seedlings inoculated with P. cinnamomi, there was a sharp reduction in the development of new roots, as shown in other studies (Robin et al 2001;Moralejo et al 2009). In contrast, after inoculation, Q. suber seedlings developed a larger number of secondary roots; these were longer, but thinner than those in control and had a lower overall weight.…”

“…Infection seems to trigger a decrease in leaf water potential, inoculated seedlings having lower values than controls. Hence, we confirm that infection by P. cinnamomi is associated with a reduction in water potential in both Q. ilex (Robin et al 2001) and Q. suber (Luque et al 1999). Other authors suggest that the stress suffered by seedlings due to flood-drought cycles also contributes to the reduction in water potential, although to a lesser extent than infection (Robin et al 2001).…”

“…Hence, we confirm that infection by P. cinnamomi is associated with a reduction in water potential in both Q. ilex (Robin et al 2001) and Q. suber (Luque et al 1999). Other authors suggest that the stress suffered by seedlings due to flood-drought cycles also contributes to the reduction in water potential, although to a lesser extent than infection (Robin et al 2001). On the basis of our results, however, we refute the idea that the decrease in water potential is exclusively due to water stress due to flood/drought cycles as indicated by other authors (Dawson and Weste 1984), and instead, attribute it to a marked deterioration of the root system.…”

“…Young actively growing plants with a high proportion of absorbing roots are particularly sensitive, as this type of root is the most affected (Marçais et al 1996). Hence, measurements related to secondary roots are considered to be effective for detecting infection by this pathogen (Robin et al 2001) and exploring differences in its effect between species, populations or genotypes (Cahill et al 1993;Belisario et al 2009). At the time of infection, a high density of inoculum is a necessary condition; in addition, infection depends on various environmental factors (Moralejo et al 2009).…”

“…Several studies have shown that this pathogen aggressively attacks Q. ilex and Q. suber (Rodriguez Molina et al 2002;Moralejo et al 2009;Serrano et al 2012b;Sánchez et al 2005), while it does not seem to cause severe lesions in other Quercus species (Marçais et al 1996). In general, inoculating Quercus species with this pathogen induces a reduction in leaf water potential and losses in active secondary roots (Robin et al 2001;Luque et al 1999), but the susceptibility of the root system to the disease varies by host species (Moralejo et al 2009). …”