Numbers Of Oospores Research Articles

Reassessment of apparent sterol requirement for sexual reproduction in Phytophthora.

Moderate numbers of oospores were produced by cross-inducing Phytophthora parasitica on chemically defined media with agar. However, even in the presence of sterols very few oospores were produced in liquid media or when agar was replaced by highly purified agarose. Elimination of agar from defined media also reduced the numbers of oospores produced by self-inducing Phytophthora cactorum in defined media. A new chemically defined medium was developed and used to assess sterol requirement for sexual reproduction in Phytophthora. Addition of lecithin to liquid basal medium greatly increased the numbers of oospores produced by P. parasitica, whereas β -sitosterol at 20μg/ml was ineffective. Lecithin in basal medium also increased oospore production by P. cactorum 550 times over basal medium only, much more than the increase by β-sitosterol. Addition of β-sitosterol at the concentrations of 1, 5, 10 and 20μg/ml to basal medium supplemented with lecithin did not increase the numbers of oospores produced by both P. parasitica and P. cactorum. Vegetative growth of both fungi in basal medium with lecithin was fast and similar to that in V-8 juice medium, but was slow in basal medium with β-sitosterol. Chromatographically purified lecithin from soybean was still very effective in promoting oospore formation, but lecithin from egg yolk and two synthetic lecithins tested were ineffective. The results show that sterols are not essential for sexual reproduction in both P. parasitica and P. cactorum.

Hyperparasitism of oospores of Phytophthora megasperma var. sojae

Hyperparasites of oospores of Phytophthora megasperma Drechs. var. sojae Hildb. were present in each of 15 field soils tested. Maximum numbers of oospores parasitized ranged from 42.5 to 87.5% for flooded soils, and from 25.5 to 73.0% for soils adjusted to 50% water holding capacity; the mean for all soils was 51.5%. The frequency of hyperparasitism was not correlated with the disease potential soils for Phytophthora root-rot of soybean as determined in seedling tests on flooded soil samples. Of eight isolated hyperparasitic fungi tested in steamed soil, the most efficient parasites were Hyphochytrium catenoides, Humicola fuscoatra, and Pythium monospermum, each of which parasitized at least 76% of oospores during 3 weeks. Hyphae were not parasitized by any of the eight fungi. Parasitism by H. catenoides in sterilized soil increased as soil temperature increased from 16° to 28°C. Parasitism by P. monospermum was maximum at 20°–24°C. Oospores of P. meyasperma var. sojae race 7 were more resistant to infection by hyperparasites than were oospores of races 1 and 3. Oospores produced in culture were slightly more susceptible to hyperparasitism in soils than were oospores produced in soybean seedlings.

Heterothallism in Bremia lactucae

It is shown that Bremia lactucae is capable of regular and predictable production of large numbers of oospores in lettuce tissues. Many isolates, while incapable of sexual reproduction when cultured alone, produced oospores in large numbers when cultured in combination with certain other isolates. This demonstrates the existence of heterothallism. In a survey of 39 isolates only two compatibility types were identified. These findings are discussed in relation to previous reports that sexual reproduction in B. lactucae is sporadic or does not occur at all. There was an inverse relationship between the intensities of sexual and asexual sporulation.

Differential effects of cholesterol in mating thalli of Pythium sylvaticum and Phytophthora capsici

Hyphae of male and female isolates of Pythium sylvaticum grown from inoculum blocks with and without 0.01 g/liter cholesterol met and produced sexual unions in water agar. Fewest oospores formed when sterol was omitted from both male and female sides. Sterol added to the male side only did not cause a significant increase in percentage of mature oospores formed in sexual unions, but addition of sterol to the female side resulted in greatly increased numbers of oospores. When two pairs of isolates were mated and cholesterol was added to both sides, a further increase in oospore formation occurred that was significant at the 5% level with one pair and at the 1% level with another pair. These results are viewed as a dosage effect due to the relative amounts of sterol carried in the male and female game-tangia and not to a specific effect on either sex. Rare mature oospores were found in single cultures of the female type isolate of P. sylvaticum. Oospore formation in crosses of isolates N14 and N58 of Phytophthora capsici increased significantly when either isolate was grown from a medium containing 0.03 g/liter cholesterol. These results are viewed as effects of bisexual mating by both Phytophthora isolates.

An Evaluation of the Excised Root Tip Method for Determining the Resistance of Pisum sativum to Aphanomyces euteiches1

Abstract Resistance in Pisum sativum to Aphanomyces euteiches was evaluated in the laboratory by the number of oospores formed in excised root tips inoculated with zoospores of the pathogen. Significantly lower mean numbers of oospores formed in root tips of 3 moderately resistant breeding lines than in those of 3 susceptible cultivars. Results were reproduceable if test procedures were adequately controlled. A sample size of 40 root tips gave 95% confidence in detecting a 40 oospore/root tip difference between means of resistant and susceptible genotypes. Lower mean numbers of oospores formed in excised root tips of genotypes which were resistant in greenhouse tests. Inoculum concentrations of 10 to 100 zoospores/plant caused severe disease in susceptible cv. New Era, while greater amounts of inoculum were needed to cause comparable disease in 2 resistant breeding lines.

The sterol requirement of Phytophthora cactorum.

SUMMARY: Several sterols and related substances were tested for ability to promote sexual reproduction in Phytophthora cactorum. These compounds were classified as active (oospores formed), partially active (oogonia and antheridia formed, but no oospores), or inactive (no oogonia). Quantitative differences between active substances were noted in respect of the numbers of oospores produced and the time of their appearance. The most active substances tested were 29-isofucosterol and β-sitosterol. Loss of activity was associated with shortening of the side chain of the sterol molecule or change in its configuration, absence of a double bond in the B ring or of a hydroxyl group at position 3 on the A ring.

PINK ROT OF THE POTATO

SUMMARY.1. A survey of the occurrence and importance of pink rot of the potato is given. The disease is widespread in Britain, but is only occasionally of serious economic importance.2. The nature of pink rot is explained and it is shown how the disease may be caused by more than one pathogenic fungus. Phytophthora erythroseptica Pethybr. is the normal pathogene, but two other species of Phytophthora have been recorded as causing the disease in the field, viz. P. drechsleri Tucker in the U.S.A. and P. megasperma Drechsler in Northern Ireland.3. The disease may be systemic in the potato plant, causing “misses” and “pink rot wilt” in the crop; normally, however, it is of importance only as a disease of the tuber.4. Under conditions of high humidity and poor aeration the disease may cause heavy loss in an affected crop during storage.5. Under normal conditions in the field the tuber becomes infected via the mother stolon; if conditions in the field or store are sufficiently moist, however, the tuber becomes infected directly, generally through the “eyes.”6. Varietal susceptibility of the potato has been considered on the three bases: (a) artificial infection of the tuber with P. erythroseptica, (b) natural infection of the tuber when stored in contaminated soil, (c) susceptibility in the field when planted in contaminated soil. Altogether fifty‐one varieties (including nine mentioned as susceptible by Pethybridge and which were not tested) have been considered, all of which proved to be more or less suceptible. No variety was found to be particularly resistant.7. Large numbers of oospores are produced by P. erythroseptica in the haulms and stolons and also to some extent in the roots of the dead potato plant. These oospores become disseminated and greatly increase the prevalence of the fungus in the soil.8. It has been shown that, in the absence of the host plant, the fungus may persist indefinitely in the soil. This persistence is apparently principally due to the resting spores, but the fact that the fungus may exist to some extent as a saprophyte in the soil cannot be ruled out.9. The disease may be satisfactorily controlled in practice by paying attention to the general principles of crop hygiene. Satisfactory crops can be grown in contaminated soil, provided that strongly sprouted tubers are planted and that soil drainage is sufficiently good to prevent the land from becoming unduly wet in times of heavy rainfall. Early harvesting of the crop tends to secure a minimum percentage of affected tubers at the time of digging. Prevention of the spread of the disease during storage may be obtained by ensuring dry and well aerated conditions in the store.The writers wish to express their very sincere thanks to Mr A. Murray, Culnafay, Toomebridge, County Antrim, who has given every facility for the carrying out of field experiments on his farm, and to the Potato Inspectors in Northern Ireland who assisted in recording occurrences of the disease.