Morphological, Molecular and Pathological Characterization of Phytophthora pseudocryptogea Associated with Rosmarinus officinalis Dieback in Tuscany, Central Italy

A severe dieback of rosemary (Rosmarinus officinalis L.) plants was observed in a medicinal/culinary herb plantation in Casole d’Elsa, Siena, central Italy. Symptoms included stunted growth, crown desiccation, root rot, collar rot and internal tissue necrosis, strongly indicative of Phytophthora root and crown rot syndrome. Morphological and molecular identification (ITS and Cox1 sequencing) of strains isolated from symptomatic stems, roots and soil revealed the occurrence of two Phytophthora species: Phytophthora pseudocryptogea, which constituted 94% of isolates obtained from the stem, root apparatus and rhizosphere; and Phytophthora megasperma, which was not recovered from plant organs or tissue, being exclusively isolated from rhizosphere soil samples at a low isolation rate (6%). The pathogenicity of the obtained strains was assessed by inoculating eighteen-month-old R. officinalis plants in a soil infestation trial. Plants inoculated with P. pseudocryptogea strains died 10 days after artificial inoculation. P. pseudocryptogea was subsequently re-isolated from the roots of inoculated, symptomatic plants, thus fulfilling Koch’s postulates. Plants inoculated with P. megasperma strains were in good vegetative condition and did not show any visible symptoms, suggesting P. megasperma to be nonpathogenic. Artificial inoculation tests thus confirmed P. pseudocryptogea to be the aetiological agent responsible for the death of R. officinalis plants in the plantation under study. This is the first report of root, collar and crown rot caused by P. pseudocryptogea on R. officinalis in Italy. There is evidence that poorly drained soils and climate constraints facilitate the spread of this oomycete. These findings highlight the critical role of nursery trade in the introduction of Phytophthora species in agroecosystems and emphasize the need for more stringent control measures.