Phytophthora Parasitica Dastur Research Articles

CAPACIDAD ANTAGÓNICA DE Trichoderma spp. NATIVA CONTRA Phytophthora parasitica Y Fusarium oxysporum AISLADOS DE CULTIVOS DE JAMAICA

La enfermedad Pata Prieta en jamaica (Hibiscus sabdariffa L.) es ocasionada principalmente por Phytophthora parasitica Dastur y Fusarium oxysporum. Estos patógenos causan pérdidas de producción en al menos 50 % en el estado de Guerrero, donde se siembra la mayor superficie de este cultivo en México. Los fungicidas que se emplean para el control de la enfermedad son caros, tóxicos y residuales, por lo que es necesario buscar alternativas de control dentro de un contexto de agricultura sustentable. El objetivo de este estudio fue evaluar la capacidad antagónica in vitro de cepas nativas de Trichoderma spp. contra P. parasitica y F. oxysporum. Las confrontaciones duales entre patógenos y agentes antagónicos se evaluaron en cajas Petri bajo un diseño experimental completamente al azar con cuatro repeticiones. Las variables registradas fueron días al primer contacto entre hifas, zona de intersección, tipo de antagonismo y porcentaje de inhibición. La información fue sometida a análisis de varianza y prueba de comparación de medias (Tukey, P ≤ 0.05). Las cinco cepas de Trichoderma sobrecrecieron a P. parasitica y cubrieron 100 % del medio; en el caso de la confrontación con F. oxysporum, Trichoderma creció sólo en dos terceras partes del medio. En las zonas de intersección se observó variación de 0.0 a 4.8 cm entre patógenos y antagónicos. La confrontación entre T. inhamatum cepa 14 y P. parasitica mostró el mayor contacto entre hifas con 4.82 cm; Ti14, Ta10 y Ta9 mostraron el primer contacto entre hifas a los 3 y 4 d en P. parasitica y F. oxysporum; la primera cepa tuvo un porcentaje de inhibición superior con 95 y 85 % para los dos patógenos. Las cepas Ti14 y Ta10 mostraron mayor antagonismo contra ambos patógenos por el tiempo al primer contacto, esporulación, crecimiento sobre los patógenos y porcentaje de inhibición. Las cepas nativas de Trichoderma representan una alternativa viable para el control de la pata prieta en Jamaica.

CAPACIDAD ANTAGÓNICA DE Trichoderma spp. NATIVA CONTRA Phytophthora parasitica Y Fusarium oxysporum AISLADOS DE CULTIVOS DE JAMAICA

The black leg disease in roselle (Hibiscus sabdariffa L.) is mainly caused by Phytophthora parasitica Dastur and Fusarium oxysporum Schltdl. These pathogens cause losses in production of at least 50 % in the state of Guerrero, where the largest area of this crop is planted in Mexico. Fungicides used to control this disease are expensive, toxic and residual; thus, it is necessary to look for control alternatives within a context of sustainable agriculture. The objective of this study was to evaluate the in vitro antagonistic capacity of native strains of Trichoderma spp. against P. parasitica and F. oxysporum. Dual confrontations between pathogens and antagonistic agents were evaluated in Petri dishes under a completely randomized experimental design with four replications. The traits recorded were days at the first contact between hyphae, intersection zone, type of antagonism and percentage of inhibition. The information was submitted to analysis of variance and comparison test of means (Tukey, P ≤ 0.05). The five strains of Trichoderma overgrew P. parasitica and covered 100 % of the medium; in the case of the confrontation with F. oxysporum, Trichoderma grew only in two thirds of the medium. In the intersection zones, a variation of 0.0 to 4.8 cm was observed between pathogens and antagonists. The confrontation between T. inhamatum strain 14 and P. parasitica showed the largest contact between hyphae with 4.82 cm; Ti14, Ta10 and Ta9 showed the first contact between hyphae at 3 and 4 d in P. parasitica and F. oxysporum; the first strain had a higher inhibition percentage with 95 and 85 % for both pathogens. The Ti14 and Ta10 strains showed greater antagonism against both pathogens by time at first contact, sporulation, growth on the pathogens and percentage of inhibition. Trichoderma native strains represent a viable alternative for black leg control in roselle.

QTLs and eQTLs mapping related to citrandarins\u2019 resistance to citrus gummosis disease

BackgroundPhytophthora nicotianae Breda de Haan (Phytophthora parasitica Dastur) causes severe damage to citrus crops worldwide. A population of citrandarins was created from the cross between the susceptible parent Citrus sunki Hort. Ex Tan. and the resistant parent Poncirus trifoliata (L.) Raf. cv. Rubidoux, both parents and two reference rootstocks (Rangpur lime and Swingle citrumelo) were grafted in a greenhouse on Rangpur lime. Inoculations were performed at 10 cm and 15 cm above the grafting region and the resulting lesions were evaluated by measuring the lesion length 60 days after inoculation. As control, non-inoculated plants of each genotype were used. In addition, we evaluated the expression of 19 candidate genes involved in citrus defense response 48 h after pathogen infection by quantitative Real-Time PCR (qPCR). We mapped genomic regions of Quantitative Trait Loci (QTLs) and Expression Quantitative Trait Loci (eQTLs) associated with resistance to P. parasitica in the linkage groups (LGs) of the previously constructed maps of C. sunki and P. trifoliata.ResultsWe found disease severity differences among the generated hybrids, with lesion lengths varying from 1.15 to 11.13 mm. The heritability of the character was 65%. These results indicate that there is a great possibility of success in the selection of resistant hybrids within this experiment. The analysis of gene expression profile demonstrated a great variation of responses regarding the activation of plant defense pathways, indicating that citrandarins have several defense strategies to control oomycete infection. The information of the phenotypic and gene expression data made possible to detect genomic regions associated with resistance. Three QTLs and 84 eQTLs were detected in the linkage map of P. trifoliata, while one QTL and 110 eQTLs were detected in C. sunki.ConclusionsThis is the first study to use eQTLs mapping in the Phytophthora-citrus interaction. Our results from the QTLs and eQTLs mapping allow us to conclude that the resistance of some citrandarins to the infection by P. parasitica is due to a favorable combination of QTLs and eQTLs transmitted by both parents.

Genetic tools and strategies for citrus breeding aiming at resistant rootstocks to gummosis disease

Phytophthora nicotianae Breda de Haan (syn. Phytophthora parasitica Dastur), causal agent of citrus gummosis disease, has caused great damage to citrus orchards throughout the world. While chemical and horticultural measures do not guarantee the preventive or curative control of citrus gummosis, the use of resistant rootstocks is the most reliable management strategy against the disease. Aiming at the development of citrus rootstocks resistant to gummosis and to better elucidate the Phytophthora-citrus pathosystem, citrus breeding programs have been ongoing worldwide, mostly employing directed crosses. These studies have succeeded in identifying differences in symptom development between resistant and susceptible rootstocks, as well as in the progeny of their crosses. In addition, differentially expressed genes were assessed, which ultimately should lead to the identification of markers involved in resistance to P. nicotianae. In this review we summarize the current knowledge of the molecular basis of citrus gummosis and the main strategies employed to obtain genetically resistant rootstocks.

Evaluation of chitosan as an inhibitor of soil-borne pathogens and as an elicitor of defence markers and resistance in tobacco plants

In in vitro assays, a chitosan polymer caused differential growth inhibition of the following pathogens isolated from tobacco: Phytophthora parasitica Dastur var. nicotianae (Ppn), Pythium aphanidermatum (Edson) Fitzp, Rhizoctonia solani Kühn, and Sclerotium rolfsii Sacc. The most sensitive were P. aphanidermatum and S. rolfsii, the growth of which was fully inhibited at a chitosan dose of 1.5 g/L; the growth of Ppn was fully inhibited at 2 g/L. In vivo assays involving plants grown from seeds immersed in chitosan, as well as plants sprayed with this product, were performed to detect the induction of defence response markers in the leaf and consequent resistance to disease. Although defence/resistance marker enzyme activities varied, activation was greater in the chitosan-treated plants than in controls. Marker enzyme activities in the sprayed plants were generally equal to or stronger than those recorded in the seed immersion-treated plants, except for phenylalanine ammonia-lyase activity at the lowest immersion concentration tested. Although there were no statistical differences among treatments with respect to resistance against Ppn, the greatest protection was afforded by the spray treatments, in which the infection index was reduced between 17 and 19% compared to the controls. In conclusion, this chitosan polymer directly inhibited the growth of several tobacco pathogens and caused the induction of defence enzymes in leaves, but was not able to protect tobacco plantlets against Ppn infection via the activation of induced resistance. This work demonstrated the potential of chitosan in protecting tobacco plants against soil-borne pathogens.

Strategies of attack and defense in plant–oomycete interactions, accentuated for Phytophthora parasitica Dastur (syn . P. Nicotianae Breda de Haan)

Oomycetes from the genus Phytophthora are fungus-like plant pathogens that are devastating for agriculture and natural ecosystems. Due to their particular physiological characteristics, no efficient treatments against diseases caused by these microorganisms are presently available. To develop such treatments, it appears essential to dissect the molecular mechanisms that determine the interaction between Phytophthora species and host plants. Available data are scarce, and genomic approaches were mainly developed for the two species, Phytophthora infestans and Phytophthora sojae. However, these two species are exceptions from, rather than representative species for, the genus. P. infestans is a foliar pathogen, and P. sojae infects a narrow range of host plants, while the majority of Phytophthora species are quite unselective, root-infecting pathogens. To represent this majority, Phytophthora parasitica emerges as a model for the genus, and genomic resources for analyzing its interaction with plants are developing. The aim of this review is to assemble current knowledge on cytological and molecular processes that are underlying plant-pathogen interactions involving Phytophthora species and in particular P. parasitica, and to place them into the context of a hypothetical scheme of co-evolution between the pathogen and the host.

Avaliação in vitro de genótipos de citros a Phytophthora parasitica

Este trabalho teve como objetivo avaliar in vitro a reação de porta-enxertos de citros (Citrus spp.) a Phytophthora parasitica. As plântulas foram cultivadas em meio MS por 40 dias sendo, submetidas a fotoperíodo de 18 h, à temperatura de 25 ºC. A inoculação foi realizada através da inserção de agulha infestada com micélio de P. parasitica. A avaliação foi realizada aos 15 dias após a inoculação, medindo-se o comprimento das lesões em centímetros. O delineamento experimental foi inteiramente casualizado, com 15 repetições. Os resultados estão de acordo com as reações de campo dos genótipos e podem ser de grande utilidade em trabalhos envolvendo resistência varietal e seleção precoce de plantas.

Structural studies of the extracellular β- d-glucans from Phytophthora parasitica Dastur

Several extracellular glucans have been isolated from Phytophthora parasitica Dastur, a phytopathogenic fungus of the carnation. These polysaccharides consist of a mixture of (1→3)(1→6)-β- d-glucans whose molecular masses varied from 1×10 4 to 5×10 6 Da. All of these polysaccharides have a main chain of β-(1→3)-linked d-glucose residues substituted with mono-, di- and oligo-saccharidic chains attached through (1→6) linkages.

Isolation and characterization of inositol sphingophospholipids from Phytophthora parasitica Dastur.

Several inositol sphingophospholipids (ISPL) were isolated from mycelia of Phytophthora parasitica Dastur, a phytopathogenic fungus of carnation. The ISPL structures were determined by fast atom bombardment. All ISPL consisted of ceramides linked to inositol phosphate. We investigated the effect of growth conditions on the ISPL produced in four different media that are commonly used for fungal cultures. We showed that P. parasitica Dastur synthesized four major classes of compounds with molecular weights of M(r) = 751, 807, 835, and 849 containing the 16:1 base and the 16:0 or 20:0 or 22:0 or 22h:1 N-acyl group. The relative abundance of the different ISPL is dependent on growth conditions.

Aluminum Amendment of Potting Mixes for Control of Phytophthora Damping-off in Bedding Plants

Control of preemergence damping-off caused by Phytophthora parasitica Dastur was investigated on three bedding plant species in a 1 peat: 1 vermiculite medium (v/v) limed at 3 kg·m–3 and drenched with aluminum at 10, 25, or 50 meq Al/100 cm3 medium. Aluminum as Al2(SO4)3 was applied as a drench at 0.75, 1.9, or 3.75 g/150 ml water to the surface of infested medium in 650-cm2 plug trays (1300-cm3 tray volume). All concentrations of aluminum were effective in controlling preemergence damping-off of snapdragon (Antirrhinum majus L.) and vinca (Catharanthus roseus G. Don, Madagascar periwinkle), but only 50 meq Al+3/100 cm3 medium was effective for petunia (Petunia ×hybrida Hort. Vilm.-Andr.). At 4 days after seeding and drenching with aluminum sulfate, exchangeable aluminum was 0, 0.5, and 2.03 meq Al+3/100 g medium, respectively, for the three concentrations used. Control of damping-off of snapdragon and vinca with 10 meq Al+3/100 cm3 medium with no detectable exchangeable aluminum 4 days after application suggests that P. parasitica was suppressed by aluminum early in the host–pathogen interaction, whereas petunia was susceptible to damping-off for a longer period before seedling emergence. Aluminum was not phytotoxic to vinca, snapdragon, or petunia grown in a limed medium.

A Novel Method to Monitor Root Decomposition Demonstrates that Salt Stress Can Enhance the Rate of Tomato Root Decomposition

AbstractThis study quantified the effects of an abiotic stress, salinity, and a biotic stress, phytophthora root rot, on root mortality and decomposition in tomato (Lycopersicon esculentum Mill.). Rhizotron windows were used to measure root mortality rates in mature, container‐grown plants. A novel assay was developed to monitor root decomposition by quantifying root‐length disappearance over time using petri dishes in incubation chambers. Phytophthora root rot infection was induced by salt stress in the presence of Phytophthora parasitica Dastur inoculum; however, the moderate levels of disease induced had no effect on rates of root mortality or on root decomposition. Genotypic differences in root decomposition under salt stress were shown by the decomposition assay, the first report of intraspecific variation in root decomposition rates. High‐salt UC82B had 50% higher root mortality and a threefold higher initial root decomposition rate than low‐salt UC82B. In contrast, salinity did not enhance root mortality or decomposition in cv. CX8303. UC82B grown under salt stress had thin roots of high N concentration, whereas salt stress did not alter N concentration in CX8303 roots. On a plant by plant basis, root N concentration was correlated with initial rootlength loss in the degradation assay, whereas root diameter was not. The results indicate that an indirect consequence of salt stress could include rapid root decomposition.

BIOLOGICAL CONTROL OF DAMPING-OFF FUNGI OF AGOHO (CASUARINA EQUISETIFOLIA L.) USING ANTAGONISTIC BACTERIA

A series of laboratory and nursery experiments were conducted specifically to determine the efficacy of 85 strains of Bacillus species and 15 actinomycetes against six fungal pathogens isolated from damped-off agoho. These damping-off fungi were: Fusarium oxysporum Schet., Rhizoctonia solani Kuhn., Phytophthora parasitica Dastur, Pythium debaryanum Hesse, and two unidentified pathogens temporarily designated as Unk 1 and Unk 2. Preliminary test using the agar-plug technique revealed that 18 of the bacterial isolates could suppress two or more of the six damping-off fungi. Fusarium oxysporum was inhibited by 17 bacterial isolates, R. solani by 8 isolates, P. parasitica by 14 isolates and P. debaryanum by 15 isolates. The unidentified damping-off fungi Unk 1 and Unk 2 were inhibited by 13 and 9 isolates, respectively. Further screening using the agar-diffusion method disclosed that 10 isolates were effective antagonists with Bacillus subtilis (Code No. R060), Bacillus sp. (Code No. R071), and Streptomyces sp. (Code No. R086) as the consistent and most effective inhibitors. Application of the three most promising antagonistic bacteria as seed treatment s howed that they effectively inhibited the growth of the damping-off fungi in the laboratory as exhibited by an increase in percent germination. Bacillus subtilis however, was not able to antagonize the effect of P. debaryanum in this particular experiment. Seed germination and seedling survival were likewise improved with the application of the three most promising antagonistic bacteria as seed treatment. This was shown after three months under nursery conditions. There were possible mechanisms of control by the antagonistic bacteria against the damping-off fungi. The mycelium and spores of the pathogenic fungus may have been attacked and parasitized by the antagonist when they were simultaneously grown in culture media. There must have been a competitive interaction between the two microorganisms. Any essential requirement of microorganisms can serve as potentially possible basis for competition. Another was antibiosis which is an inhibitory effect exerted by an organism upon another organism through the production of antibiotic compounds. Moreover, several strains of bacteria are effective in lysing cell walls of pathogenic fungi under laboratory conditions. Lysis is often attributed to production of cell wall degrading enzymes like chitinase and gluconase that may hydrolyze major constituents of fungal cell walls.

Distribution ofPhytophthora parasiticavar.nicotianaeRaces and Their Sensitivity to Metalaxyl in Georgia

Tobacco (Nicotiana tabacum L.) samples displaying typical black shank symptoms were collected in 1990-1992 throughout the tobacco-producing area of Georgia. Cultures of Phytophthora parasitica Dastur var. nicotianae (Breda de Haan) Tucker (Ppn) isolated from samples were stored in culture until the fall of each of the years. The tobacco cultivars K 326 (low Ppn resistance), Speight G 70 (moderate Ppn resistance), Coker 371 Gold (high Ppn resistance), and the line 1071 (immune to race 0, susceptible to race 1) were grown in a greenhouse and used to determine virulence and races. Isolates were cultured on toothpicks impregnated with V8 juice agar, and the toothpicks were pushed into the stems of test plants to inoculate them

Short- and Long-term Heat Stress Effects on Phytophthora Root Rot of Hibiscus

Roots of hydroponically grown Hibiscus Rosa-sinesis L. cuttings were exposed to 22, 30, 40, or 50C for 20 minutes, after which they were inoculated with zoospores of Phytophthora parasitica Dastur. Visual assessment of root discoloration and culturing of randomly selected root pieces 10 to 13 days after treatment showed that roots exposed to 40 or 50C had a significantly higher incidence of infection than those exposed to 20 or 30C. Plants were also grown in pots containing University of California (UC) mix or washed, graded sand and exposed to solar radiation for 1 day or 3 weeks, respectively. Root systems of plants in direct sunlight heated to 52C, while roots of shaded plants heated to 40C. Assessment of infection severity was done visually or by means of a Phytophthora-specific antibody probe. In all experiments, infection severity was highest in sun-exposed plants and was insignificant to moderate in shaded plants.

Effect of growth conditions on the structure of β- d-glucans from Phytophthora parasitica dastur, a phytopathogenic fungus

Several glucans have been isolated from the cell walls of Phytophthora parasitica, a phytopathogenic fungus of carnation. These polysaccharides consist of a mixture of (1 → 3), (1 → 6)-β- d-glucans whose molecular masses varied from 7·5 × 10 3 to 2 × 10 6 daltons. All these polysaccharides have a main chain of β-(1 → 3)-linked d-glucose residues. They differ in the presence of 1 → 6 branched oligosidic chains with 1, 2 or 4 d-glucose residues. The effect of various growth conditions on the nature of these glucans have been studied. When asparagine and a high concentration of glucose were present in the culture medium and when the culture was shaken, the production of the lowest molecular weight polysaccharides and their degree of branching were increased.

Genetic Analysis of Resistance to Phytophthora Root Rot in Tomato (Lycopersicon esculentum Mill.)

The resistant accession, LA1312, and the susceptible cultivar ‘Peto 343′, were crossed to develop F1, F2 and BC1 populations for genetic analysis of resistance in tomatoes to Phytophthora parasitica Dastur, the causal agent of Phytophthora root rot. There was no maternal effect on resistance. Generation means analysis indicated that tolerance to Phytophthora root rot was under genetic control with both simple (additive and dominance) and digenic interaction (additive × additive and additive × dominance) effects contributing to the total genetic variation among generation means. Weighted least square regression analysis indicated that the majority (ca. 96 %) of the genetic variation could be explained by simple additive effects alone. Narrow sense heritability was estimated as 0.22. Based on effective factor formulae, at least five effective factors controlled the resistance. Implications for breeding procedures are discussed.

Phytophthora Root Rot and Irrigation Schedule Influence Growth and Phenology of Processing Tomatoes

Abstract Processing tomatoes (Lycopersicon esculentum Mill.) grown in field plots with soil infested or not infested with Phytophthora parasitica Dastur. were furrow-irrigated for 4 to 8 hr every 14 days (normal) or 28 days (less frequent) or with alternating 4- to 8-hr and 24-hr irrigations every 14 days (prolonged). Disease developed more rapidly and symptom severity was significantly greater on shoots and roots of plants that received prolonged irrigations. Disease symptoms on roots progressed more rapidly and were observed earlier than those on shoots in plants given prolonged or normal irrigation. Severe disease during early vegetative and reproductive growth caused significant reductions in total plant, leaf, and fruit dry matter and in the numbers of flowers and fruit in plants receiving either prolonged or normal irrigation. Diseased plants given prolonged irrigation also partitioned less total dry matter into leaves and fruit and more into stems than noninoculated plants. Less frequent irrigation of infested plots caused a delay in disease onset and reduced the impact of disease on numbers of flowers, fruit, and dry matter accumulation. Phytophthora-induced water stress during critical stages of crop development apparently can have major impacts on plant growth, phenology, and yield in processing tomato.

In vitro production of specific polysaccharides: isolation and structure of an antitumor active β-glucan from Phytophthora parasitica

Polysaccharides are known to be an important group of biological polymers which are of great interest for industrial use. Besides their physical properties, their chemical structures can be the base for specific applications in medicine. Various polysaccharides are known to exhibit an antitumor activity, which is thought to be host mediated by a stimulation of the host's immune system. A β-1.3/1.6-glucan P isolated from the cell walls of Phytophthora parasitica Dastur (Mating type A1, 1828) exhibited strong antitumor activity against Sarcoma-180 in mice. Growth of the fungus was markedly stimulated by β-sitosterol and calcium ions. The fungus was shown to be thiamine heterotroph. At optimal growth conditions up to 30 g (dry weight) of fungal mycelium could be harvested within 5 days in 81 of culture medium, which represented ~6 g of cell wall material. A specific water-soluble cell wall glucan P was isolated by extraction with hot water and purification by ion-exchange and Con A chromatography. By methylation analysis it was shown that the β-glucan consisted of 1.3-, 1.3.6- and terminal-glucose residues in a molar ratio of 5:1:1. Sequential Smith degradations combined with methylation analyses showed the presence of mono- as well as di- and trisaccharides as side chain constituents in a ratio of 9:2:3. A structure of the antitumor active β-1.3/1.6-glucan P is proposed.

Alternaria Panax on Umbrella Tree.

The umbrella tree (Schefflera actinophylla (Endl.) Harms. (synonym: Brassaia actinophylla Endl.), which is indigenous to humid coastal Queensland north from about the Tropic of Capricorn, is a popular garden tree as well as being grown as a potted indoor and outdoor plant in southern Queensland. In June 1981 potted umbrella tree plants growing in a Brisbane nursery were severely affected by a leaf spot and leaf blight disease. Leaf symptoms ranged from very small light brown necrotic spots to large irregular dark brownlblackish necrotic areas (Fig. 1). Leaf spots were often surrounded by chlorotic zones. Severely affected plants were defoliated. A species of Alternaria occurred on lesions, and was consistently isolated. The fungus was subsequently identified as Alternaria panax Whetzel. This identification was confirmed by Dr S.M. Francis of the Commonwealth Mycological lnstitute (IMI 269850; BRlP 13804). Pathogenicity tests were performed by spraying a spore suspension (1 x 105/ml) from cultures derived from a single spore isolate of the fungus onto healthy plants of S. actinophylla and S. arboricola (Hayata) Merrill (dwarf umbrella tree). Control plants of each species were sprayed with sterile water. All plants were enclosed in plastic bags for 3 days at 20°C. After 6 days the temperature was raised to 26OC. Symptoms identical with those seen in natural infections developed on the S. actinophylla plants within 4 days. S. arboricola symptoms were quite different. Small light brown necrotic spots (round to irregular) surrounded by a diffuse chlorotic zone were present on mature leaves 12 days after inoculation. On the undersurface of the leaves the spots were much more prominent, appeared corky, and were slightly raised. On soft immature leaves, small irregular dark red spots were produced and again they were much more prominent when viewed from the undersurface. Immature leaves were distorted and spots also appeared on petioles. However, there was no defoliation. A. panax was successfully reisolated from all inoculated plants. With S. arboricola percent recovery was always lower than with S. actinophylla and with time the fungus became harder to isolate. Alternaria panax is an important pathogen of many plants in the family Araliceae. Recorded hosts include Schefflera actinophylla, S. arboricola, Dizygotheca elegantissima (Hort. Veitch) R. Vig. & Guillaum, Tupidanthus calyptratus Hook. f. & T. Thomas, and plants in the genera Aralia, Panax, Polyscias, Pseudopanax and Meryta (1, 2, 3). Many of these plants are grown as ornamentals in Queensland. A. panax has been recorded on Araliaceae in New Zealand, Spain, U.S.A. and Venezuela (2). The fungus has recently been recorded on Polyscias fruticosa (L.) Harms. in Queensland. Phytophthora parasitica Dastur (P. nicotianae B. de Haan var. parasitica (Dastur) Waterh.) has been reported to cause a leaf spot on S. actinophylla (4) similar to that caused by an Alternaria species. In Queensland Phytophthora palmivora (Butler) Butler has been found on one occasion causing a leaf spot of S. actinophylla nursery plants. Symptoms were very similar to Alternaria leaf spot on this host. To my knowledge this is the first recording of Alternaria panax of S. actinophylla in Queensland.

Mating-Type Distribution of Phytophthora colocasiae on the Island of Hawaii

Heterothallic species of Phytophthora require the presence of opposite mating types known as A1 and A2 of the same or different species for oospore formation (12). Recent evidence suggests the regulation of this type of sexual reproduction by sex hormones (8). Phytophthora cinnamomi Rands, an unspecialized root pathogen (6), is widespread on the island of Hawaii (7). The mating-type distribution of this fungus isolated from the island is one-sided. All 47 isolates obtained from soils and plant roots were of the A2 type (7). A further study showed that all 252 isolates of P. cinnamomi from roots of various plant species collected at 21 locations throughout the island were also A2 type (Ko, unpublished data). Two isolates from macadamia cankers were the only record of A1 type from this island (3). Phytophthora colocasiae Rac., the causal organism of leaf blight of taro [Colocasia esculenta (L.) Schott], is also heterothallic and common in taro fields on the island of Hawaii. The fungus is a specialized foliar pathogen (5, 14). Because of its great contrast to P. cinnailmomi, the mating-type distribution of P. colacasiae throughout the island of Hawaii was investigated. Pieces of taro leaves and petioles infected with P. colocasiae were placed on the selective medium developed by Masago et al. (10). After a 3-da incubation at 24 C, mycelia from the edges of P. colocasiae colonies were transferred to V-8-agar plates (10% V-8 juice, 0.02% CaCO3 and 2a% agar). The mating type of each isolate was determined by pairing with an A1 type (P991) and an A2 type (P731) of Phytophthora parasitica Dastur. Both isolates of P. parasitica were supplied by G. A. Zentmyer.