First Report of Phytophthora cryptogea Root and Crown Rot on Peach Trees in Chile.

Abstract

Phytophthora cryptogea Pethybridge and Lafferty was consistently isolated from the Nemaguard rootstock (Prunus persica × P. davidiana) of peach trees in Chile. Symptoms included reddish necrotic tissues at the base of the trunk often extending to the main roots, root rot, gummosis, foliar chlorosis, lack of vigor, and dieback, with severely infected trees dying. Isolations with corn meal agar (Difco, Detroit, MI) amended with antibiotics and fungicides (ACMA) (2) and pure cultures were obtained by hyphal tip transfers to ACMA. Ten isolates from different locations were identified by morphology and growth at cardinal temperatures (1). These isolates produced hyphal swellings and sporangia by using carrot juice broth (2) for 48 h followed by the addition of 1% (w/v) nonsterilized soil extract. Sporangia were nonpapillate, internally proliferating, noncaducous, ovoid to obpyriform, and with mean dimensions of 36 × 24 μm. All isolates were typed as A1 using a P. cinnamomi A2 culture as the test isolate and produced oospores on clarified V8 juice agar amended with thiamine, tryptophan, and β-sitosterol (2) after 15 to 30 days at 20°C in the dark. Mycelia grew between 5 and 30°C, with optimal growth at 20°C and no growth at 35°C. The ITS sequence analysis performed by IMI (CABI Bioscience, Wallingford, UK) indicated a close match with P. cryptogea, P. drechsleri, and P. erythroseptica, however, peach isolates were differentiated from P. drechsleri and P. erythroseptica by the absence of mycelial growth at 35°C and the heterothallic production of oospores, respectively. Four P. cryptogea isolates were pathogenic when inoculated onto 1-year-old twigs detached from a Nemaguard tree, causing reddish necrotic lesions varying from 7 to 29 mm long after 15 days of incubation at 20°C. Two-year-old P. persica var. nectarina cv. Ruby Diamond budded on Nemaguard and cultivated in 100-liter containers in open field conditions were inoculated with 500-ml of a mycelial suspension (4.5 × 106 propagules per ml) per plant added to 15-cm-deep holes around the trunk. Plants were immediately irrigated and maintained at field capacity during the experiment. An equal number of noninoculated trees treated similarly were left as controls. During the spring, plants developed a general chlorosis and declined within 90 days postinoculation. Tests were repeated with similar results. In other preliminary tests, plants developed cankers characterized by gumming and reddish necrotic lesions underneath the bark following trunk inoculations with mycelium in agar of the same isolates obtained on ACMA. These tests were also repeated. The pathogen was consistently reisolated from symptomatic tissue. To our knowledge, this is the first report of P. cryptogea as a pathogen on peach trees in Chile.