Phytophthora cactorum

Phytophthora cactorum is a plant pathogen that causes root rot on rhododendron and many other species. Phytophthora cactorum has an extremely wide host range, and can infect over 200 species or 160 genera of trees, ornamentals, and fruit crops. In general, P. cactorum is capable of infecting both young and old plants, and causes root rots and crown rots of the many genera it infects. Although the symptoms this pathogen produces varies between the types of organisms it infects, in general disease occurs during periods that are both wet and warm. Furthermore, most infections are caused by zoospores entering the plant through wounds. On older trees, P. cactorum causes the formation of sap exuding dark colored cankers on the trunks of trees, as well as leaf size and number reduction, chlorosis, and dieback of branches. The diagnosis of a P. cactorum infection of trees, is based on the identification of symptoms, in particular the oozing cankers, and confirmation of symptoms in a diagnosis lab or utilization of a field ELISA detection kit. P. cactorum can be a major problem in apple orchards, as it can cause crown, collar, and root rots in apple trees. When infecting apple trees, the organism can attack through wounds either above or below the soil line, impairing phloem and root function, and causing stunting, foliar disorders, and death after several years. Furthermore, because the pathogen causes damage to the phloem of the tree, one diagnostic method is to check for necrotic phloem tissue at the base of the tree which will be orange to red-brown in the early stages and dark brown in the later stages of infection. A good example of P. cactorum causing foliar disease is on ginseng. Foliar disease of ginseng usually occurs during May and early June, causing the leaves to become transparent and papery. Ginseng foliar infection occurs through the rain splash dispersal of spores from the soil onto above ground wounds. Once infected, P. cactorum works its way down to the roots, rotting them and killing the plant. P. cactorum is also one of the causal agents of black rot of orchids. When infecting orchids, this organism first produces small black lesions on the pseudobulbs of the orchid, which then enlarge and may engulf the entire pseudobulb, leaf, or move through the rhizomes to other portions of the plant prior to killing it. Diagnosis of orchid black rot by P. cactorum is through the identification of lemon shaped zoosporangia with either a papilla or a short pedicel, the presence of oospores, or molecular identification. Since there are multiple species of Phytophthora that are capable of causing disease on orchids, classification only to the genus level is required for proper prescription of disease management techniques. Crown rot or root rot of strawberries is a common example of diseases of fruit crops caused by P. cactorum. On strawberries, P. cactorum infects the roots and the base of the plant causing stunting and reduced leaf size, with possible complete plant collapse later in the season. Similarly to apple trees, crown rots of strawberries caused by P. cactorum can be partially diagnosed by cutting the crown of the plant and observing brown vascular tissues, and root rots by brown or black stunted roots. p. cactorum is a homothallic (only having one mating type, can mate with itself) oomycete, and displays the right angled mycelial branching with a constriction at the base of the branch, which is highly characteristic of other Phytophthora species. Within the hyphae, they have singular diploid nuclei that are regularly spaced. In addition, young hyphae only have cross walls separating reproductive parts; however older hyphae may have cross walls anywhere. Furthermore, although the hyphae are not the main survival unit of P. cactorum, as long as they are not completely desiccated, they are capable of surviving until just above freezing temperatures. P. cactorum produces one sexually produced survival spore called an oospore, and one asexually produced survival spore called a chlamydospore. Oospores are double-walled and uninucleate during dormancy, but become multinucleate in preparation for germination. In contrast, the chlamydospores have only one resistant wall and are multinucleate at all stages. Chlamydospores are larger than oospores in size, and are only formed under certain environmental triggers. The trigger for preferential formation of chamydospores over oospores can be either large temperature or moisture oscillations.