Some microorganisms can inhabit the internal tissues of a plant without causing sapparent harm. These organisms are called endophytes. Some endophytes, however, are capable of rapid growth and of causing physiological disturbance of the plant under certain circumstances, causing disease. Phaeoacremonium and Phaeomoniella spp associated with Petri disease (Young Vine Decline) have been observed to behave as latent endophytes, capable of inhabiting the internal vine tissues without causing disease symptoms. It is important to establish what are the environmental and physiological factors responsible for triggering their pathogenic behavior. Field observations suggest that conditions of stress for the vines are involved in inducing the occurrence of disease.
Our research project investigates the endophytic biology of Phaeoacremonium spp and Phaeomoniella chlamydospora, focusing on the capacity of penetration and invasion of grapevine tissues and on the effect of stress factors in the pathogenic response of these fungi.
Isolation results and scanning electron microscopy observations showed that P. inflatipes and P. aleophilum are capable or penetrating uninjured roots and shoots. Light microscopy observations of paraffin embedded sections showed that rapid spread of the fungus in injured-inoculated roots was through the vascular tissues and intercellular spaces of the cortex. In injured-inoculated shoots, rapid spread apparently occurred through the intercellular spaces of the pith. Spores of P. aleophilum were observed in the xylem vessel. This is the first documentation of the presence of spores of Phaeoacremonium spp in the xylem of a grapevine with young vine decline.
Artificial inoculation of grapevines with P. inflatipes in greenhouse experiments has shown that the fungus did grow into the shoots although it was abundant at the stem tissue right below them.
Field experiments have been set up to study the effect of different stress factors on predisposing the plants to Petri disease. Stress factors being evaluated are: root configuration, early fruiting, fertilizer treatments, and water deficit stress. These are long -term experiments that will be evaluated over several years. In the first year of application, two foliar and one soil drench fertilizer treatments did not have an effect on the vegetative growth of Merlot/101-14 vines.
The composition of plant vascular sap is known to change under stress conditions. We have compared the effect of sap from water stressed vines on the growth of Phaeoacremonium spp and Phaeomoniella chlamydospora with the effect of sap from non-stressed vines. The experiments indicated that sap from water stressed vines enhances growth of P. inflatipes in vitro, with respect of sap from non-stressed vines. There was no differential effect on the growth of P. chlamydospora.