Crown Rot and Sudden Death of Almond Trees Caused by Oomycetes in Southern Iberian Peninsula

Abstract

From March 2020 to November 2022, 12 commercial almond (Prunus dulcis) orchards showing symptoms of chlorosis, wilting, defoliation, cankers, crown rot and profuse trunk gummosis were surveyed in southern Spain and Portugal. Diagnostic tests were performed by isolating oomycetes from affected tissues and by DNA extraction from root and wood tissues and PCR amplification of the rDNA internal transcribed spacer (ITS) genomic area using the specific primer pair Phni-F/Phni-R. A total of 62 isolates were collected: 18 Phytophthora-like isolated from plants and 44 Pythium sensu lato-like from waterlogged soil. Seven representative isolates were selected and identified by multilocus sequencing of the ITS and cox2 genomic regions and phylogenetic analysis. The following oomycetes were identified: Phytophthora niederhauseri, Phytopythium litorale, Globisporangium heterothallicum, G. macrosporum, G. paroecandrum and G. violae, with Ph. niederhauseri being the most frequent. The effect of temperature on the mycelial growth of seven isolates was evaluated in carrot agar and corn meal agar, with Ph. niederhauseri and Pp. litorale showing the highest optimal growth temperature (around 28°C–29°C) regardless of the culture media used. Pathogenicity tests of seven isolates were conducted on almond plants of cv. Guara grafted on Garnem rootstock. Ph. niederhauseri isolates were the most aggressive, causing crown rot, gummosis and plant death starting 3 weeks after inoculation. Pp. litorale showed intermediate aggressiveness, while Globisporangium species developed small cankers and minimum lesions, not causing plant death. Based on pathogenicity tests, Ph. niederhauseri is considered the true pathogen of the disease, while the Pythium sensu lato-like isolates must be considered secondary pathogens in waterlogged conditions.