Behavior Of Zoospores Research Articles

Aggregation of Phytophthora capsici Zoospores and their Interaction with Zoospores of P. palmivora

Zoospores of Phytophthora palmivora Butler aggregated without encysting within 2 min when they were suspended in water (Ko & Chase, 1973). Such zoospore behaviour was temperature-dependent with an optimum temperature of 16 °C. Aggregate formation also depended on both zoospore concentration and depth of suspension. We now report a similar behaviour of zoospores of Phytophthora capsici Leonian in water. The interaction between zoospores of P. palmivora and P. capsici was also investigated.

Factors affecting zoospore responses towards stomata in hop downy mildew ( Pseudoperonospora humuli) including some comparisons with grapevine downy mildew ( Plasmopara viticola)

When zoospores of Pseudoperonospora humuli in water were inoculated in the light on to abaxial surfaces of hop leaf disks, which had been maintained in light, they began to respond to stomata within 2 min. After 4 min a proportion significantly greater than random had settled on stomata. Typically, only one zoospore was associated with each stoma. In darkness, zoospores remained motile for longer periods and showed no preference for stomata, finally settling on the leaf surface at random. Zoospores of Plasmopara viticola in light settled on stomata of grapevine leaf disks in groups of up to 10 but, in darkness, on comparatively few stomata in groups of up to 28. Two processes are involved in the typical response of zoospores of P. humuli to stomata in the light, a shortening of their motile period on the leaf surface and their selection of stomata as sites of settling. Analysis of zoospore behaviour on hop leaf disks subjected to various light and chemical treatments and on plastic replicas of leaves with open or closed stomata showed that two stimuli, one chemical, involving photosynthesis, and the other physical, from open stomata, contribute independently to the complete response. Whereas both chemical and physical stimuli encouraged zoospores to select stomata, only the chemical stimulus caused their motile period to be shortened. Various aspects of the zoospore response to stomata of leaf disks and replicas are illustrated in scanning electron micrographs.

The influence of stomatal opening on the infection of hop leaves by Pseudoperonospora humuli

In experiments using both attached hop leaves and leaf disks floating on water, a general correlation was found between the number of stomata per unit area open at the time of inoculation with Pseudoperonospora humuli and the amount of subsequent infection, assessed as area infected and degree of sporulation after 7 days. Whether stomatal opening was varied by light or darkness, chemicals or different soil moisture régimes, a similar result was consistently obtained, irrespective of leaf age or cultivar. Microscopic examination of the leaf surface after inoculation in the light with a large number of open stomata, revealed that the majority of zoospores settled and encysted singly on stomata which were then penetrated by germ tubes. In contrast, in darkness, when stomata were mostly closed, zoospores settled between stomata and few germ tubes encountered and penetrated stomata. This differential response to stomata occurred on hop leaves of various ages and cultivars and on leaves of certain non-hosts. A statistical test, based on sequential analysis, allowed zoospore-stoma associations to be evaluated with the low zoospore numbers used. In reciprocal inoculations of hop and vine leaves zoospore behaviour in Pseudoperonospora humuli was compared with that in Plasmopara viticola. Marked differences in the number of zoospores settling on single stomata occurred which appeared to be due to both stomatal and zoospore factors. The results are discussed in relation to downy mildew in the field, resistance to the disease and the possible mechanism of the zoospore response.

Comparative Morphologic and Taxonomic Studies in the Genus Thraustochytrium

SUMMARYThe genus Thraustochytrium consists of 10 species. Characters used to separate these taxa include: 1) zoospore and zoosporangial size; 2) nature of the rhizoids; 3) proliferation characteristics; 4) discharge methods; 5) pigmentation of thalli in mass; and 6) wall thickness of reproductive bodies. Although these taxa appear distinct when various aspects of the above characters are used, comparative morphological studies on a Thraustochytrium roseum—T. visurgense complex and a T. motivum—T. aureum complex point up the difficulty in using many of the characters which presumably separate the thraustochytriaceous taxa. The difficulty arises because the characters are susceptible to variation, the characters tend to overlap, and the characters of the isolates of this study, in conjunction with the characters of the already established taxa, form a character gradient or cline.More specifically, pigmentation, sporangial discharge characteristics, zoospore behavior, rhizoidal nature, and wall thickness, characters used to separate Thraustochytrium roseum and T. visurgense, are either highly variable or clinal so as to suggest that the two species are in fact but a single taxon, T. roseum. Also some of the characters used to distinguish T. aureum and T. motivum are so variable or clinal that their value is questionable.

BEHAVIOR OF PRIMARY AND SECONDARY ZOOSPORES OF SAPROLEGNIA SP.

The behavior of primary and secondary zoospores of Saprolegnia sp. was studied by phase microscopy. In particular the fate of their flagella upon encystment was examined. It was shown that primary zoospores withdraw their flagella, whereas secondary zoospores shed their flagella upon encystment.

A New Species of Petersenia Parasitic on Pythium

SUMMARYA new species of Petersenia, P. utriculoba (Phycomycetes, Lagenidiales, Olpidiopsidaceae) parasitic in Pythium intermedium de Bary, isolated from dry soil collections, is described. Sporangia of this parasite have the form of irregularly-shaped vesicles consisting of a varying number of pouch-like or lobulate swellings extending in several planes. Resting bodies, described here for the first time in the genus Petersenia, are located in saccate structures similar in form to the sporangia. The life-history of this parasite as determined from pure and unifungal cultures is given. A discussion concerning zoospore behavior in the family is given.