H. H. Ho

Pythium vexans causing patch canker of rubber trees on Hainan Island, China

This is the first report of patch canker disease of rubber trees (clone RRIM600) in China. It is characterized by discrete irregular patches of rotted, discolored bark and wood, accompanied by a decrease in latex flow. A total of seven isolates of Pythium vexans were obtained from the diseased bark of the trunks and roots of rubber trees. Inoculating these isolates into healthy, mature rubber trees resulted in symptoms similar to patch canker and the same fungal species was re-isolated from the diseased tissues. This is also the first record of Py. vexans in Hainan.

Effect of hymexazol on growth and reproduction of low temperature Phytophthora species

The effect of hymexazol (3-hydroxy-5-methylisoxazole or HMI) on growth and reproduction of low temperature Phytophthora species with growth maxima under 30 °C, was studied on clarified V-8 juice agar medium at 50 μg/ml and 5 μg/ml. HMI severely inhibited growth of P. infestons, P. phaseoli, P. porri and P. syringae, retarded the growth of P. hibernalis and P. lateralis but enhanced the growth of P. fragariae and P. ilicis. Phytophthora cactorum and P. pseudotsugae with a maximal temperature of 30 °C were moderately inhibited.HMI reduced or prevented sporangial production on agar medium by P. cactorum, P. hibernalis, P. ilicis, P. infestons, P. lateralis, P. phaseoli, P. porri and P. primulae, and suppressed chlamydospore formation in P. porri and to a lesser extent, P. lateralis. The production of sex organs was prevented by HMI in P. phaseoli, P. porri, and P. syringae, reduced in P. hibernalis and P. ilicis but unaffected in P. cactorum, P. primulae and P. psuedotsugae.

Affinity groups among plant pathogenic species of phytophthora in culture

Cluster analysis of the plant-pathogenic species ofPhytophthora in culture was conducted based on the morphological, physiological and pathological characters. The dendrograms of similarity coefficients produced five distinct affinity groups broadly in agreement with those established by Waterhouse (1963). The merging of Group V and Group VI of Waterhouse was proposed.

Phytophthora spp. from rubbertree plantations in Yunnan Province of China

Phytophthora spp. associated with leaf fall, stem canker and black stripe of rubbertree (Hevea brasiliensis Muell.-Arg.) in Xishuangbanna of Yunnan Province were isolated from leaves, bark, fruit and soil in the plantations. Of a total of 50 isolates, 42 were designated as P. citrophthora (R. E. Sm. & E. H. Sm.) Leonian, while the others were identified as P. palmivora (Butl.) Butl., P. colocasiae Rac. and P. cactorum (Leb. & Conn) Schroet. This appears to be the first report of P. citrophthora on Hevea brasiliensis.

A synopsis of the occurrence and pathogenicity of Phytophthora species in mainland China.

To date, 26 species of Phytophthora have been reported in mainland China but only 23 are accepted as good taxa. Phytophthora nicotianae (P. parasitica) is the single most important species causing over 40 different plant diseases; P. boehmeriae is widespread attacking Gossypium, Boehmeria, Citrus, Broussonetia papyrifera and Pterocarya stenoptera; P. “fragariae var. oryzo-bladis” is unique to mainland China causing blight of Oryza sativa seedlings; P. megasperma sensu lato has been isolated from Glycine max, Asparagus officinalis and Spinacia oleracea. In Hainan Province, P. heveae is present in the soil without causing apparent diseases to Hevea brasiliensis there.

Pythium dimorphum from rhododendron

A pythiaceous phycomycete was isolated from diseased roots of rhododendron in New York. It was characterized by fast growth, preference for high temperature for growth, ovoid to elongate, non-papillate, internally proliferating sporangia and large spherical, pigmented chlamydospores, but failed to produce sex organs in single or dual cultures. It was identified as Pythium dimorphum Hendrix & Campbell. This appears to be the second report of its occurrence since its first isolation from lolly pine roots in Louisiana in 1971.

Phytophthora spp. associated with the decline of Cedrus deodara in Nanjing, China.

Recently, 3-year-old seedlings of Cedrus deodara in Nanjing, China developed root rot, crown rot, chlorosis and defoliation, often resulting in death. Two species of Phytophthora were isolated. One was identified as A2 mating type of P. cinnamomi Rands while the other failed to produce sex organs in single or dual cultures and was tentatively assigned to P. drechsleh Tucker. Their pathogenicity was confirmed by re-inoculation and re-isolation.

Electron microscope studies of oogonial wall and elemental composition of oogonia in Phytophthora

Scanning electron microscopy of oogonia of Phytophthora spp. showed that the oogonial wall was smooth in P. cactorum, P. citricola, P. heveae, and P. palmivora; finely granular in P. megasperma and P. megasperma var. sojae; and coarsely granular in P. parasitica. Transmission electron microscopy demonstrated that the oogonial wall in Phytophthora was composed of three layers with the middle layer being the least or the most electron dense. A coat of amorphous material was found on the entire outer surface of the oogonial wall. Elemental analysis of oogonia by means of a SEM electron probe microanalyzer revealed similar emission spectra among Phytophthora spp. with a characteristic peak for calcium.

The occurrence ofPhytophthora heveae in mainland China.

Phytophthora heveae was isolated from soil in Hainan Island. This is the first report of its occurrence in mainland China. The world literature onP. heveae is also reviewed.

The nature of paragyny in Phytophthora

The nature of paragyny was studied in Phytophthora cactorum, P. citricola, P. hibernalis, P. megasperma, P. porri and P. syringae. In most cases, the gametangial initials arose in pairs, side by side, in close proximity, on the same or different hyphae. The encirclement of an oogonial initial by an antheridial initial occurred infrequently only. The paragynous antheridium was attached to the surface of an oogonium by an adhesion layer of dense, amorphous material. The real difference between paragyny and amphigyny appeared to be the way in which the antheridium and oogonium were affixed together to achieve sexual fusion. Amphigyny was considered the more advanced system which evolved along with heterothallism to ensure successful outbreeding.