NAME OF THE ORGANISM: Phytophthora cactorum (PHYTCC)

Not candidate

Phytophthora cactorum has a cosmopolitan distribution and a reported host range of more than 200 species, spanning at least 154 genera of vascular plants within 54 families (e.g. Erwin & Ribeiro, 1996), including herbaceous and woody plants, e.g. Cydonia, Malus and Pyrus (Grígel et al., 2019; Garmaroodi et al., 2023), Juglans regia (Mircetich & Matheron, 1983), Prunus dulcis (Pérez-Sierra et al., 2010), P. armeniaca, P. avium, P. cerasus, P. domestica, P. persica (Grígel et al., 2019), and Prunus salicina (Plantwise Knowledge Bank, 2022).
Although traditionally considered to be a generalist, marked differences of P. cactorum isolates occur in degree of pathogenicity to different hosts. When P. cactorum isolates from different host plants were tested on a common host, significant differences of virulence were observed (Chen et al., 2023). For example, the strains isolated from hosts other than strawberry are not able to cause crown rot symptoms on strawberry (Eikemo et al., 2004), but all pathotypes were able to cause strawberry leather rot (Nellist et al., 2021; Seemüller & Schmidle, 1979). It was found that P. cactorum isolates from strawberry crowns were less pathogenic on apple barks than isolates from strawberry fruit or apple and vice versa, with apple and strawberry fruit isolates being less pathogenic on strawberry crowns (Nellist et al., 2021; Seemüller & Schmidle, 1979, Chen et al., 2023).
Although Phytophthora cactorum can be transported with plants for planting (e.g. rootstocks Carisse & Khanizadeh, 2006), it is foremost a soilborne pathogen. The pathogen was also isolated from soil from unaffected orchards and forest soil samples (Turechek, 2004 in Garmaroodi et al. 2023). The most important propagules concerned with survival of P. cactorum are oospores, which are formed as a result of sexual reproduction, and the asexually produced chlamydospores. Both of these structures can survive for years in orchard soils (Erwin & Ribeiro, 1996). Predisposing factors, such as waterlogging or planting of trees on sites not suitable for the species, as well as contributing factors which either reduce the vitality of the tree (e.g., extreme droughts or defoliations) or favor the pathogen (e.g., excess soil moisture following heavy rain, flooding or irrigation), can accelerate the disease process or make it possible in the first place (Jung et al., 2018).
Irrigation can sometimes contribute to development of Phytophthora diseases, both by spreading the pathogens over long distances through surface water supply networks of rivers and canals and by favoring production, dispersal, and infection by the pathogens’ zoospores (Browne et al., 2012; Felix-Gastelum & Mircetich, 2005 – cited from Brown, 2017).
Because of the wide host range and longevity of inoculum sources in the environment, planting material are not considered to be a significant pathway for introduction under outdoor field conditions. Plants for planting produced under at-risk situations could only be considered a significant pathway if the intended use was for a protected facility that used uninfested soil media and had been thoroughly cleaned of potential infection sources before use.