Analysis of symptom expressions and transmission rates caused by the plant pathogen Phytophthora ramorum on native chaparral plants from the genus Arctostaphylos

Phytophthora ramorum is the causal agent of Sudden Oak death (SOD), ramorum dieback and ramorum leaf blight which affect both forest environments and nurseries. This oomycete pathogen has had a huge economic impact on the nursery and lumber industry. Forests in California have experienced substantial mortality of oaks affecting the forest dynamics and diversity. Our research investigates four native species and two ornamental cultivars of plants, which belong to the genus Arctostaphylos (manzanita) and are considered to be new hosts for P. ramorum in the chaparral ecosystem of California. Symptom expression and transmission rates were analyzed on Arctostaphylos glauca, A. uva-ursi, A. silvicola and A. pumila in field experiments and in growth chamber experiments. Additionally, symptom development on A. densiflora Howard McMinn and A. densiflora Sentinel cultivars was also studied in growth chamber experiments. Foliage and roots of A. glauca, A. silvicola, A. pumila and A. uva-ursi were inoculated with P. ramorum at the National Ornamental Research Site in Dominican University of California (NORS-DUC). Symptoms were not observed on the foliage and roots of the inoculated manzanitas in the NORS-DUC. The inoculated manzanitas in the growth chamber expressed the presence of P. ramorum. Genetic analysis of the necrotic foliage of all four Arctostaphylos spp. and symptom expressions on two cultivars of Arctostaphylos densiflora confirms the susceptibility of the hosts to ramorum blight in the growth chamber. Sporulation of the pathogen was studied on a symptomatic California Bay Laurel tree (Umbellularia californica) which plays a key role in the disease epidemiology. The aerial transmission from a symptomatic plant to healthy plants was monitored on all four manzanita species by placing them in the immediate vicinity of the diseased Bay Laurel. No sporangia were detected in the air and symptoms of ramorum blight were not expressed on the manzanitas. Although field experiments did not detect P. ramorum on the Arctostaphylos spp., in the growth chamber experiments all six species expressed ramorum blight and dieback symptoms confirming their susceptibility to P. ramorum. Native manzanitas may be are at risk if P. ramorum establishes itself on the chaparral ecosystem of California