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Remarks (public):For a complete description including images see www.cababstractsplus.org/dfb 
Remarks (internal):Distinguished from P. citricola (CMI Descript. 1143) by the pedicellate, deciduous sporangia with the thick protruding apex as well as by other cultural and serological characteristics (Waterhouse, 1957; Burrell et al., 1966). Collar rot of apple has been known in the U.S.A. since 1858 but, as elsewhere, was confused with other diseases and injuries until the work of Baines (1939) who showed that P. cactorum was the primary pathogen. Another early record of disease losses caused by P. cactorum was the destructive blight and root rot of ginseng in Japan (Hori, 1907), later reported from the U.S.A. by Whetzel (1920) and recently recorded causing serious losses to this crop in the U.S.S.R. (37: 499, 40: 120). Control through the use of resistant varieties has received considerable attention, particularly in apple (33: 609; 39: 328; 45, 1425, 2530). Juvenile resistance has been found (Schwinn, 1965) but susceptibility to zoospore infection increases with age both in the tree (35: 685) and the fruit (45, 2166). Dormant trees are more resistant to inoculation than those in active growth (22: 30, 31). Resistance has also been found in soybean (43, 311), sweet clover (22: 27) and walnut (10: 214; 38: 39). Chemical control has been reported with bordeaux (18: 807; 22: 170; 37: 499), captan (38: 214) and brestan (41: 93). There have also been attempts at chemotherapy of bleeding canker of maple and birch with helione orange
(21: 104, 394) and biological control using soil amendments and green manures (43, 1698; 45, 2167).
 
Description type:Non-original description 
Description:Phytophthora cactorum (Lcb. & Cohn) J. Schrot., Kryptogamenfl. Schlesien 3(1): 236, 1886.
= Phytophthora omnivora de Bary, Bot. Ztg 39: 619, 1881.
Hyphae normally c. 6 µm wide but may be irregularly swollen though without characteristic hyphal swellings. Chlamydospores formed by some strains under certain conditions, up to 55 (av. 33) µm diam.; wall 1-1,5 µm thick. Sporangiophore a regular unichasial sympodium, close in moist air (with very short stalks), more elongated under water, slender (0,5-1 µm wide) with a slight swelling et the base of each branch but not elsewhere on the sporangiophore; cross wall delimitating the sporangium 3-4 µm down the pedicel and bearing a plug. Sporangia abundant on the usual solid media, broadly and regularly ellipsoid or ovoid to obpyriform, 36-50 (-55) x 28-35 (-40) µm, apex with a conspicuous hemispherical papilla with apical thickening up to 5 µm deep; deciduous with a pedicel up to 4 µm long occluded by the septal plug. Sex organs abundant on most media. Oogonia (19-) 25-32 (-38) µm diam., spherical or tapering to the base; wall thin, colourless or slightly yellow. Oospore definitely aplerotic, usually 20-26 µm diam.; wall colourless, 2 µm thick. Antheridia nearly spherical to irregularly clavate, 15 (-21) x 13 µm, nearly always applied close to the oogonial stalk, often obscured in a knot of hyphae, nearly always paragynous and monoclinous. Culture usually slightly radiate with uniform slight aerial mycelium (but see Stamps, 1953); min. temp. about 2°C, opt. (20-) 25 (-28)°C, max. c. 30°C.
Hosts: On a very wide range of host plants embracing 54 families (particularly Rosaceae) and over 150 genera (Nienhaus, 1960; Rangaswami, 1962) including ash, beech, cherry, conifers, apple, pear, apricot, strawberry, cucurbits, eggplant, cacti, gooseberry, rhododendron, lilac, ginseng, rhubarb, avocado, birch, maple and oak.
Diseases: Damping-off of seedlings including ash, beech, cherry and conifers; fruit rot of apple, pear, apricot, strawberry (leather rot), cucurbits and eggplant; leaf and stem rot of cacti, gooseberry, rhododendron, lilac, ginseng and rhubarb; collar rot and crown rot of apple and other fruit trees stem canker of avocado, birch, maple and oak; root rots in general.
Geographical distribution: Widespread in tempetate areas (CMI Map 280).
Physiological specialization: Strains showing variation in pathogenicity have been reported in isolates from apple (Baines, 1939; Welsh, 1942; Smith, 1955; Sewell & Wilson, 1959), onion (Stamps, 1953) and gooseberry and pea (Smith, 1955). Mutations in pathogenicity in single zoospore cultures have been reported by Stamps (1953) to occur spontaneously to apple, and by Buddenhagen (1958) to peach induced by X-rays. More frequently isolates have been found capable of attacking a wide range of unrelated hosts (Nienhaus, 1960; 22: 458; 17: 399; 44, 3388; 9: 390; 39: 142; 4: 740; 10: 214). Isolates showing some tolerance of copper have been obtained by Nienhaus (1959).
Transmission: Soil-borne. Widely distributed in soils of apple orchards affected with collar rot (35: 685; 37: 132). Strawberry fruit in unmulched beds and wind-fall apples and pears on the ground frequently become infected by contact with infested soil (4: 101; 35: 685) where the pathogen may occur to a depth of 50 cm and persist for as long as 15 yr. in ploughland previously under apple orchards (42: 75). This has been partially explained by the irregular germination of oospores which require a dormant period (Blackwell, 1943; Legge, 1952). Apple fruit and gooseberry shoot infection are most frequent on lower branches subject to rain-splash which carry zoospores (41: 49). Long periods of rain, heavy dews, fog and high soil moisture favour the spread of the pathogen (39: 250; 45, 2553; 22: 30-31; 42: 391).
Literature: Blackwell & Waterhouse, Trans. Br. mycol. Soc. 15: 294-321, 1931; Waterhouse, Trans. Br. mycol. Soc. 15: 311-321, 1931; Blackwell, Trans. Br. mycol. Soc. 26: 71-89, 1943 (life history); Waterhouse, Mycol Pap. 92, 1963 (taxonomy); Sewell & Wilson, Ann. appl. Biol. 53: 275-280, 1964 (cf. P. syringae); Burrell, Clayton, Gallegly & Lilly, Phytopathology 56: 422-426, 1966 (serological distinction from P. citricola); Rangaswami, Pythiaceous Fungi, 1962; Nienhaus, Phytopath. Z. 38: 33-68, 1960 (host range); Baines, J. agric. Res. 70: 11-30, 1939; Smith, Orchard. N.Z. 28: 16-17, 19, 21, 1955; Stamps, Trans. Br. mycol. Soc. 36: 248-254, 1953; Sewell &Wilson, J. hors. Sci. 34: 51-58, 1959; Welsh, Can. J. Res. 20: 457-490, 1942; Buddenhagen, Am. J. Bot. 45: 355-365, 1958 (physiologic specialization); Joustra, LandbDocumtie 20: 3-7, 1965 (46 refs); Schwinn, Phytopath. Z. 54: 1-30, 162-184, 1965 (43 and 60 refs); Schmidle, Erwerbsobstbau 2: 169-171, 1960; Erwerbsobstbau 6: 181-183, 1964; Braun & Krober, Phytopath. Z. 32: 35-94, 1958 (74 refs) (apple); McIntosh, Phytopathology 49: 795-797, 1959; Mclntosh & O'Reilly, Phytopathology 53: 1372-1373, 1447, 1963 (pear); Wright, Beraha & Smith, Pl. Dis. Reptr 50: 283-287, 1966 (13 refs) (strawberry); Hori, Bull. Imp. Central agr. Expt Stn Japan 1: 153-162, 1907; Whetzel, Science 31: 790-791, 1910 (ginseng).

 
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