Phytophthora cactorum (apple collar rot)

P. cactorum is capable of causing pre- and post-emergence damping-off in a number of plant species. Werres (1995) reported on the capacity of P. cactorum to kill seedlings of Fagus sylvatica (beech) within a few days, and it is also responsible for reducing the number of sprouting seedlings (Madsen, 1995). P. cactorum also causes seedling blight in a range of Pinus spp. (Crandall and Hartley, 1938), Salix scoulerana (Stuntz and Seliskar, 1943) and Robinia spp. (Collinge, 1912).One of the most economically significant diseases caused by P. cactorum is crown, collar and root rot of apple, pear and other woody species (Rose and Lindegren, 1925; Baines, 1935), and reviewed by Harris (1991a). When visible symptoms are evident on the foliage, for example, stunted terminal growth and small chlorotic leaves, considerable development of root and/or crown rot has occurred. Crown rot affects the rootstock, whereas collar rot is a disease of the scion. Roots show a general necrosis, and the dead bark often has a reddish tinge in either crown or collar rot, with infection often starting at the soil line. There is often a sharp line of demarcation between reddish brown diseased tissue and the healthy white bark. It is important to make a diagnosis before secondary foliar symptoms become evident, otherwise chemical treatments are unlikely to be successful.On nursery rootstocks, infection may be present without visible symptoms. Lesions where roots emerge from the rootstock can be a symptom, as well as dark-brown, soft-rotted lesions on older roots. Isolation is necessary to confirm infection, as early symptoms of P. cactorum infection are often difficult to detect. The greatest period of susceptibility in apple and pear is during the leaf and flower development stages (Braun and Nienhaus, 1959). Wounds provide effective infection courts, although P. cactorum can also penetrate unwounded tissues.In apple and pear, fruit roots can also occur; resulting in dark-brown lesions that can rot the entire fruit. Apples are generally only susceptible at maturity, whereas pear fruit are susceptible at all stages of development.There is one record of P. cactorum being seedborne on safflower (Zad, 1992). In vegetatively propagated species such as woody tree species, potato and strawberry, P. cactorum can be readily transmitted in planting material.Cankers and crown rots of trees (dogwoods, maples, walnuts, stone fruits)Cankers can occur on the stem or trunk at or near the soil line, with discoloration of the infected bark, sometimes extending a small distance into the internal tissues. Infected trees also show a sparse, yellow-green foliage, poor twig growth and stunting (Smith and Barrett, 1931).Leathery rot of strawberriesP. cactorum can cause crown rot (red-brown lesions within the crown) and root rot of strawberries, but the most serious symptom is a fruit rot. Under favourable conditions (moist cool weather), more than 80% of the fruit can be destroyed (Madden et al., 1991). Berries at all developmental stages are susceptible. Dark-brown lesions develop on green fruit, which later turn light brown with purple edges (Rose, 1924). Diseased berries become tough and leathery, hence the name leathery fruit rot. Infected berries never show soft rot symptoms. Discoloration of the vascular tissue in green fruit is a characteristic symptom of leathery rot.

P. cactorum is unequivocally a serious pathogen of a wide range of plant species. Jeffers and Aldwinckle (1988) reported that up to 88% of grafted apple and pear planting stocks and 97% of unbudded rootstocks in commercial nurseries were infected by P. cactorum and/or P. cambivora.Because several other species of Phytophthora cause diseases of similar symptomatology to those caused by P. cactorum, it is difficult to ascribe specific levels of losses to P. cactorum, particularly for the root and collar rot diseases of apples, pear and stone fruits (Jeffers and Aldwinckle, 1988; Harris, 1991a).