Outcome of Ceratocystis platani inoculations in Platanus × acerifolia in relation to season and inoculum dose

Abstract

Ceratocystis platani is the causal agent of canker stain, the most destructive disease of Platanus spp. The selection of resistant tree genotypes should be an effective method of controlling the disease. Although breeding programs for resistance have been developed, there is no validated protocol for an in depth evaluation of the resistance phenotype. Testing the variables to identify the conditions that fully challenge the genetic potential of the host is crucial not to overestimate labile resistant phenotypes. Here we report results of testing different inoculum doses and inoculation times - early and late spring, summer and autumn - on the response of susceptible plane genotypes. Late spring inoculation gave rise to the quickest death pattern occurred, followed by early spring inoculation. Nevertheless Ceratocystis platani was aggressive also in the hottest period of the summer. The capacity of C. platani to initiate canker stain in the hottest period of the year underlines its capacity to be virulent in a wide range of temperatures. Although autumnal inoculation enabled fungus entry, its progression was precociously halted as effective and stable resistance reactions were opposed by the majority of the trees. An in vitro study of mycelial growth and conidial germination confirmed the capacity of the fungus to be active or to keep the viability in a wide range of temperatures such as in the three seasons that were taken into consideration. Thus we hypothesize that the failure of symptom expression after autumnal inoculation might be due to a combination of a temperature-linked reduction in pathogen virulence, and a season-linked resistant reaction of the host. Overall our data suggest that, in the context of legal sanitary measures, only the coldest and driest periods of the year should be considered for pruning and the removal of infected trees. The different inoculum doses did not condition the death pattern. Comparing germination in suspensions at 1.000 and 10.000 conidia per µl, self-inhibition germination occurred, as the germination rate was inversely correlated with the conidia concentration. This is a first step in the definition of a resistance-testing protocol for an in depth evaluation of resistance to canker stain.

Keywords

• Inoculum Dose
• Inoculation Time
• Resistance Reaction
• Self-inhibition Conidia Germination
• Thermal Niche
• Resistance Testing Protocol