Abstract:Phytophthora drechsleri f. sp. cajani (Pal et al.) Kannaiyan et ai. causes stem and leaf blight m pigeon pea {Cajanus cajan [L.] Millsp.) in India. The asexual phase occurs in artificial culture as well as on the host tissue. Sparse oospore formation has been observeed in old cultures. A technique has been evolved in which abundant mature oospores are formed on the leaflets of pigeon pea and also on glass slides using zoospores and myceliai discs on the former but only mycelial discs on the glass slides. The largest number of oospores was formed after incubation for 36 h at 250C.
Abstract: When dark-grown mature oospores of Phytophthora cactorum were activated to germinate bv exposure to 5 uW cm-2 nm-1 of fluorescent light ai 20—220C in the presence of certam flavm inhibitors such as KI, saiicylhydroxamie acid and phenylacetic acid ai 40, 1. and 0.1 mM respectivclv, photoactivation and hence subsequent germination of oospores were inhibited without appreciable irreversible effect on oospore viability. Likew-ise, when applied during the light period, NaN3, and KCN at I mM reduced photoactivation but had a minimal effect on dark reactions. Diphenylamine, an inhibitor ot certain carotcnoids, had no effect on photoactivation ot oospores. The data suggest that the photorecL-ptor pigment for activation of oospore germination is a flavin.
Abstract: When oospores from the pairing between A1 and A2 mating types of Phytophthora infestans were treated with 0.25 % KMnO4 solution for 15 min and incubated at 19°C under light on a modified S4-L medium, germination commenced within 4 days and reached about 70 % after 20 days. Under these conditions, more than 25 % of oospores obtained from a 4-day-old culture germinated. To obtain a high germination rate of P. infestans oospores, light was essential during germination but not during growth and oospore formation. The optimum time for activation of oospores with 0.25 % KMnO4 was 15 to 30 min and a suitable concentration of KMnO4 for 15 min activation was 0.25 to 0.45%
Abstract:Phytophthora infestans is able to produce oospores in leaves of potato and tomato plants after inoculation with a mixture of Al and A2 mating-type isolates. Various conditions for oospore formation were analysed. Under controlled conditions, oospores were produced in potato leaves at temperatures ranging from 5 to 250C. In leaves of potato cultivar Bintje incubated at 150C, oogonia and antheridia were observed 6 days after inoculation and thick-walled oospores appeared 3-4 days later. In field experiments oospores were found in leaves and stems of potato cultivars Bintje, Irene and Pimpernel and in leaves, stems and fruits of tomato cultivar Moneymaker within 2 weeks after inoculation. A bioassay was developed to test the survival of oospores in soil under various conditions. To determine whether late-blight infections derived from infectious soil were caused by oospwres, DNA fingerprinting was performed. DNA fingerprint probe RG-57 was suitable for distinguishing asexual progeny from recombinant progeny arising from soil-borne oospores. We demonstrated survival of viable, infectious oospores of P. infestans in soil during the winter of 1992-93. Oospores were not infectious from soil exposed to temperatures of 400C or higher but in the range 350C to as low as — 800C for 48 h, oospores survived.
Abstract: Oospores of Phytophthora infestans were produced in potato leaf discs floating on metalaxyl solution (100 mgmL–1a.i.) and inoculated with all combinations of two metalaxyl-sensitive and two -resistant parental isolates. Numbers of oospores produced varied between different matings, depending on parents, in the absence of the fungicide and when metalaxyl was added 0, 7, 14 and 21 days after inoculation. Oospores were not produced when metalaxyl was added at the time of inoculation (0 days) when either one or both parents were sensitive to metalaxyl. In two of three such matings further oospore formation was arrested when metalaxyl was added either 7 or 14 days after inoculation. Oospores extracted from leaf discs 14, 21 and 28 days after inoculation were assessed for germination on water agar after 21 days. Germination of oospores from water control treatments varied between 6 and 30% depending on the cross. Germination was significantly reduced in oospores of metalaxyl-sensitive parents extracted 28 days after inoculation of leaf discs treated with metalaxyl 0, 7 and 14 days after inoculation compared with the 21-day treatment. Minimal differences in germination were observed for oospores from the mating of resistant parents irrespective of metalaxyl treatment, although germination was generally low, not exceeding 8·5%.
Abstract: Oospores of Phytophthora cactorum were produced in cleared V-8 broth supplemented with 30 mg/l b-sitosterol in the dark at 20–220C for 2–3 months. Environmental conditions during oospore formation or maturation such as light, low temperature and also ageing could not compensate the light requirement for oospore germination. Dark grown oospores did not infect hypocotyl of susceptible saflower seedlings, an indicator plant for the pathogen, if they were not photoactivated 3–15 days or longer prior to infection. If oospores are the sole source of survival of P. cactorum in soil, then the collar region of the plant would be the main site of infection. Thus any treatment which reduce the chance of oospore germination on the soil surface may support management of this disease.
Abstract: High moisture content of the host tissue (< 88%) and low ambient r.h. (50-54%) favoured oospore fomiation under controlled environments. It took 14-16 days for oospores to develop; thereafter the number of oospores increased with time and decreased with moisture content of host tissue. High ambient r.h. (> 80%) did not favour oospore formation under fteld or controlled eonditions. Oospore
formation was detected in inoculated plants grown in the field when the ambient r.h. declined to < 74% and moisture content of host tissue decreased to 83.7-85.6%, It took 8 days (cv. Kufri Chandramukhi) to 13 days (cv. Kufri Jyoti and Kufri Badshah) for oospores to develop. Cultivars also differed in their
response to oospore produetion, cv. Kufri Chandramukhi being more responsive (4800 oospores g-1fwt) than ev. Kufri Jyoti and Kufri Badshah (1320 and 390 oospores g-1f wt respectively). Oospores produced in vitro remained viable when buried in soil in the temperate highlands of Himachal Pradesh and sub-tropieal plains of Uttar Pradesh, India for more than 150 days, i.e. beginning of the next crop season. The oospores germinated and initiated late blight infection at the base of the stems after 21 -30 days of incubation of the potato plants raised in oospore-infested soil. It took 2 days for newly formed oospores to genninate and this delay time increased to 75-77 days after 180-days burial. It took 15 days for their germination (47%) in soil extract as compared to 50 days in sterilised distilled water.
Abstract: This paper provides the first evidence of A2 type 1 and type 2 isolates of Phytophthora cinnamomi producing selfed oospores in planta in an Australian soil and in a potting mix. Oospores were observed in infected lupin (Lupinus angustifolius) roots incubated for 7 d either in the substrate under potted Acacia pulchella plants, or in soils collected from under
and near varieties of A. pulchella in jarrah (Eucalyptus marginata) forest. The A2 type isolates varied in their ability to produce selfed oospores and none were produced by A1 isolates. The gametangial association was amphigynous and spores were predominantly spherical with diameters from 13–40 mm. The oospores were viable but dormant. Two A2 type isolates produced small numbers of selfed oospores with amphigynous antheridia axenically in Ribeiro’s liquid medium within 30 d, and one A2 type 2 isolate produced oospores after mating with an A1 strain. Evidence is presented that the presence of roots of Acacia pulchella, and particularly A. pulchella var. glaberrima and var. goadbyi, enhances the production of oospores.
Abstract: The effect of different parameters, including the type of nutrients, the quality of the gelling agent, and the genotype of the strain, were evaluated in the production of gametangia by Phytophthora ramorum in vitro. By comparing different agar sources on a carrot-based medium, a delay or a failure in the production of oospores was observed in pairings carried out on media supplemented with technical agar. In contrast, oospores were produced on other agar types, the production on media supplemented with agarose being slightly higher. The formation of gametangia was also influenced by the genotype of the strains involved in the pairing. A European A1 strain producing very few chlamydospores was found to be a better mating partner than other A1 strains. Using a carrot–agarose medium and selected genotypes, all European isolates were characterized in terms of mating type. A macroscopic experiment highlighted a particular spatial distribution of P. ramorum oospores in vitro. A method using polycarbonate membrane was evaluated to assess the selfing ability of P. ramorum.
Abstract: The formation of oospores of Phytophthora infestans was studied in tomato and potato crops and volunteer plants under field conditions, and in laboratory tests with leaf discs of potato cultivars differing in their level of race-nonspecific resistance. Oospores were readily detected in blight-affected tomato leaflets and fruits, and in leaflets of field crops and volunteer potato plants. Oospores extracted from blighted potato leaflets yielded 13 oospore-derived progeny. Oospores were also produced following inoculation of leaf discs of eight potato cultivars expressing different levels of race-nonspecific resistance with a mixture of sporangia of A1 and A2 isolates. The highest numbers of oospores were produced in cvs Bintje (susceptible) and Pimpernel (resistant), and the lowest in Nicola (intermediate resistance). The relationship between lesions per leaflet and oospore incidence, affected by varying A1 : A2 ratios, was explored using a simple mathematical model, and validated by comparing actual oospore production in leaflets with multiple lesions of the race-nonspecific-resistant potato clone Lan 22-21 with the predictions generated by the model. Survival of oospores was investigated after their incorporation in either a sandy or a light clay soil in buried clay pots exposed to the local weather conditions. Over 6 years these soils were regularly assessed for their infection potential using floating leaflets in a spore-baiting bioassay. Sandy and clay soils contaminated with oospores remained infectious for 48 and 34 months, respectively, when flooded. Infections of floating potato leaflets occurred within 84–92 h and ceased after 11 days. Soil samples remained infective if dried and re-flooded on two, but not more, occasions.
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