Abstract
An extensive survey conducted in the Saïss plain of Morocco during the 2017–2018 growing season revealed that 35 out of 50 apple and pear orchards were infested with a pathogen that causes the decline disease. Morphological and phylogenetic tree analyses using the cox II gene allowed us to identify the pathogen as Phytopythium vexans. Interestingly, no Phytophthora and Pythium species were isolated. The occurrence and prevalence of the disease varied between locations; the most infested locations were Meknes (100%), Imouzzer (83%), and Sefrou (80%). To fulfill Koch’s postulate, a greenhouse pathogenicity test was performed on the stem and collar of one-year-old healthy seedlings of apple rootstock M115. Symptoms similar to those observed in the field were reproduced in less than 4 months post-inoculation with root rot disease severity ranging from 70 to 100%. The survey results evidenced that apple rootstocks, soil type, and irrigation procedure may contribute significantly to the occurrence of the disease. The disease was most prevalent in drip water irrigation and sandy-clay soil on wild apple rootstock. Accordingly, a rational drip advanced watering system and good sanitation practices could eliminate water stagnation and help prevent the onset of this disease. It was concluded that Pp. vexans occurrence may be strongly influenced by irrigation mode and type of soil. Therefore, the obtained findings of this study could help to better understand the recurrence of this disease and to develop a reliable integrated strategy for its management.
Highlights
Several microorganisms ubiquitous in the soil have been assumed to be causal agents of apple and pear decline diseases [1], with microorganisms belonging to the oomycetes considered to be the main causal agents of these diseases [2]
The present study focuses on the distribution of the oomycete genus pathogenic to apple and pear trees in Morocco, in order to (i) identify and characterize the oomycete microorganisms associated with the disease, (ii) investigate the possible farming factors and practices allowing the occurrence of the disease, and (iii) fulfill Koch’s postulates in order to verify the role of the isolated pathogen as causative agent of the disease
After a deep investigation into the cultural practices adopted by the farmers, in particular excessive watering of trees, it was assumed that the disease was caused by oomycetes
Summary
Several microorganisms ubiquitous in the soil have been assumed to be causal agents of apple and pear decline diseases [1], with microorganisms belonging to the oomycetes considered to be the main causal agents of these diseases [2]. They are divided into several more or less virulent genera and species; the most devastating of which are the species belonging to Pythium and Phytophthora [3]. Phytophthora and Pythium species can survive for a long time in the soil and in diseased plants They grow under persistent conditions of humidity, too frequent watering, or excessive irrigation and at temperatures around 15–16 ◦ C.
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