Zoospore Concentration Research Articles

Chemotactic auto-aggregation in the water mould Achlya

SUMMARY: Free-swimming zoospores of several isolates of the oomycetous water mould Achlya demonstrate mutual attraction (adelphotaxis) which may result in auto-aggregation. In the apparent absence of exogenous chemotactic signals macroscopic globular aggregates or microscopic plaques of cysts are formed. Auto-aggregation was favoured by high concentrations of zoospores in suspension; the growth rate of spontaneous aggregates was estimated at 4--10 x 103spores min−1; growth of an aggregate was limited to about 7 min. By means of chemotactic assays, attractant activity for zoospores was detected in the vicinity of aggregates, and in zoospore or cyst supernatants. The attractant activity extracted from spore aggregates was heat-stable. In the absence of exogenous nutrients, the macroscopic aggregates of an homothallic isolate produced sporangia and oogonia. Aggregation was readily initiated as a result of zoospore chemotaxis towards capillary tubes containing exogenous attractant in an agar medium. Aggregating zoospores frequently assumed a slender swarm configuration moving directly towards the aggregate at the capillary-tube tip. Analysis of video recordings showed that many zoospores joining the swarm were attracted directly by the swarm rather than by the exogenous attractant. Such aggregation was a time-limited process, the duration of which was extended by the presence of amino acids in the zoospore suspension. Under natural conditions zoospore aggregation may be a dual-component chemotactic process triggered by exogenous attractants and amplified by chemical signals from the aggregating spores. The possible consequences of adelphotaxis and auto-aggregate formation, namely enhanced inoculum potential, syngamy and spore production, are discussed.

Specific saccharide residues are involved in the recognition of plant root surfaces by zoospores of Pythium aphanidermatum

The binding and encystment of zoospores of Pythium aphanidermatum to root surfaces of cress ( Lepidium sativum) was used as a quantitative model of fungal-plant recognition. Binding principally involves the root elongation zone of intact (undamaged) seedlings, and was standardized with respect to time, temperature and zoospore concentration to that the initial rate of zoospore binding could be assessed. Zoospore-root binding is abolished by trypsinization of zoospores or periodate oxidation of the root surface. Lectin and enzyme modification of the root surface mucilage polysaccharide reduced zoospore binding; treatments that block, or remove terminal fucosyl residues are especially effective. Incubation of zoospores with exogenous root mucilage polysaccharide also reduced zoospore binding but the effectiveness of the mucilage was reduced if pretreated with periodate or with exofucosidase. The results suggest that protein-containing receptors on the zoospore surface interact with fucose-containing ligands present in root surface mucilage to promote zoospore-binding and subsequent encystment. The involvement of other sugar residues cannot, however, be ruled out.

Factors affecting expression of resistance in pepper (Capsicum annuum) to blight caused by Phytophthora capsici in screening trials

The effects of plant age, fungus isolate, zoospore concentration and inoculation method on the resistance of pepper (Capsicum annuum) to blight (Phytophthora capsici) were studied under controlled conditions. Each of the factors studied produced differing responses in the pepper lines tested hut line CNPH 286 proved to be a reliable source of resistance and is now being used in a breeding programme. An inoculation method for screening pepper lines for blight resistance is proposed, using a standardized zoospore suspension applied to the base of plants 45 days from sowing.

SOME GENERAL OBSERVATIONS ON ZOOSPORES OF THE CHYTRID RHIZOPHYDIUM PLANKTONICUM CANTER EMEND.

SummaryGeneral microscopic observations of zoospores of the chytrid Rhizophydium planktonicum Canter emend, contained within a small glass chamber, have shown that they have strong reactions to visible light. There appears to exist a threshold level of dim light below which zoospores will eventually enter what is termed the ‘state of quiescence’ and above which they will continue to maintain active swimming. Alternations between these two states can he manipulated with ease under the microscope. Free swimming zoospores after a period in dim light or darkness and quiescent zoospores both respond to sudden exposure to bright light.The concentration of zoospores into aggregates of individuals circling at the upper and or lower surfaces of a chamber is recorded. As viewed from above, those uppermost circle in a clockwise direction while those below go round counter‐clockwise.Adherence between zoospores as swimming individuals or in the encysted state has been seen but there is no evidence of any fusion taking place. A specially adapted channel image intensifier made possible the examination of material under infra‐red.

Viable saprolegniaceae spores on the epidermis of the Salmonid fish Salmo trutta and Salvelinus alpinus

Techniques are described for monitoring Saprolegniaceae in hatchery pond water and on the epidermis of hatchery-reared brown trout Salmo trutta L. and char Salvelinus alpinus L. The water supply to the hatchery occasionally includes spores of Saprolegnia type I (taxonomically in or near the S. diclina — S. parasitica complex), which are responsible for the outbreaks of Saprolegnia infection that occur in the hatchery. The saprolegniaceae spora in the pond water increased from 200 spores/1 or less to a maximum of 22 200 spores/1 when infected fish were present in the water. This increase was solely due to Saprolegnia type I spores. Experimental applications of high concentrations of secondary zoospores of Saprolegnia type I to trout and char indicate that although the majority of spores are removed or inactivated during the first 24 h of subsequent quarantine in clean water, a small but significant number of viable spores remain on the body surface. These findings are discussed in the light of our knowledge of Saprolegnia infections of fish and in relation to the possible defence mechanisms of fish to pathogenic fungi.

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.

Aggregation of Zoospores of Phytophthora palmivora

Summary: Zoospores of Phytophthora palmivora suspended in water aggregated within 2 min at 16°C. Individual spores in aggregates were separate and actively swimming. The optimum temperature for aggregation was 16°C. Aggregate formation depended on zoospore concentration and depth of suspension.

Zoospore germination in the water mold, Blastocladiella emersonii: II. Influence of cellular and environmental variables on germination

The effects of cellular and environmental variables involved in the population assay of zoospore germination have been examined. The following results were obtained: 1. 1. Within the standard conditions employed, only growth density and, to a lesser extent, temperature, seriously affect either the synchrony or the rate of zoospore conversion to round cells. 2. 2. Synchrony of round cell formation varies as a function of growth density in DM-2, but not PYG, medium. 3. 3. Synchrony, and probably rate, of round cell formation are profoundly influenced by temperature; neither a temperature threshold nor a temperature optimum was found within the viable (growth) range. 4. 4. Neither synchrony nor rate of round cell formation is affected by various manipulations of the zoospores prior to germination (centrifugation, deflagellation, prechilling, extended periods of swimming) or by zoospore concentration (within the standard range). At high zoospore concentrations, T 50 is increased. 5. 5. Synchrony and rate of round cell formation are indistinguishable in germination solution, an inorganic salts medium, and DM-2, a defined growth medium. Round cell formation is considerably more asynchronous in the previously standard, undefined growth medium (PYG). 6. 6. Of the constituents of germination solution, only KCl is necessary for moderately synchronous round cell formation, and the degree of synchrony is markedly dependent on KCl concentration. On the basis of these and other (Soll et al., 1969) results, the following conclusions bearing on the germination process and on its control have been made: 1. 1. The moment of initiation of zoospore germination is precisely defined. 2. 2. KCl (or chloride salts of certain other monovalent cations) can initiate the changes associated with zoospore germination. 3. 3. Neither initial presence of a flagellum, flagellar retraction nor later presence of an internal flagellar axoneme are required for the successful completion of germination. 4. 4. At standard zoospore concentrations, cells do not interact to permit semisynchronous germination; at higher concentrations, cells do interact to inhibit germination. 5. 5. Zoospores can successfully complete germination at ice temperature. 6. 6. Heterogeneity in zoospore populations with respect to time of germination can be predictably varied by controlling the density of growth cultures from which the zoospores are derived.

Pythium sp. (Phycomycetes:Pythiales) pathogenic to mosquito larvae

A species of the fungus Pythium causing a high rate of mortality in a field collection of Aedes sierrensis was examined for its potential as a biological contol agent. Motile zoospores, the only infective stage, exhibited a strongly positive chemotactic response to wounds of mosquito larvae, but were apparently incapable of penetrating normal cuticle. Wide variation of mortality rate in laboratory tests was a function of harshness of physical treatment which larvae received prior to testing and zoospore concentration.

TRANSMISSION OF TOBACCO NECROSIS VIRUS BY ZOOSPORES OF OLPIDIUM BRASSICAE.

Summary: Strain d of tobacco necrosis virus (TNV) was transmitted by zoospores of 3 different isolates of Olpidium brassicae (Wor.) Dang. to roots of Mung bean and lettuce grown in modified Hoagland's solution diluted 1/20. On Mung bean roots necrotic local lesions formed one day after exposure to virus and zoospores. Virus in lettuce was assayed by inoculation to leaves of French bean. Virus transmission was favoured by decreasing salt concentration and increasing the pH value of the nutrient solution and depended also on the concentrations of virus and zoospores. With 105 zoospores/ml. transmission to lettuce was obtained with as little virus as 0.05 μg./l. When the virus concentration was 5 μg./l., 50-100 zoospores/ ml. were effective. Fungus infection as measured by the number of zoo-sporangia in the root was not strictly correlated with virus infection. Exposure of roots to virus + zoospore mixture for 1 min. sufficed to infect them with virus. More transmission occurred when virus was added before or together with zoospores, than after. Roots, exposed to zoospores for 10 min., then washed, were more readily infected by TNV when virus was introduced during the first hour or two after zoospore attachment to the root cells than later; there was some transmission even when virus was withheld till 4 hr after washing. Immersing roots, inoculated with fungus and virus, in hot water (60°) killed the fungus but not the virus, and varying the interval between inoculation and heating showed that virus became established after 2-3 hr. Isolates of Olpidium brassicae naturally contaminated by strain d or a of TNV were freed from contamination by inoculating lettuce roots with dilute zoospore suspensions. Zoospores mixed or naturally contaminated with TNV were partially separated from it by centrifugation. Virus transmission was prevented by adding concentrated homologous antiserum to zoospores that had already been exposed to virus, or by adding very dilute antiserum to virus before mixing it with zoospores. The extent to which transmission was prevented by antisera to other strains of TNV depended on the degree of their serological relationship to strain d. The present evidence does not support the suggestion that TNV is carried inside the fungus.

Physiologic races of Phytophthora fragariae

SUMMARYPathogenicity tests with forty‐nine isolates of Phytophthora fragariae, cause of red core disease of strawberry, using the strawberry varieties Huxley, Perle de Prague, Climax, Talisman, Aberdeen and Cambridge Vigour, revealed the existence of twelve physiological races of the fungus on the basis of a pathogenic or non‐pathogenic reaction. Further differentiation, on the basis of degree of pathogenicity to Perle de Prague and to Climax, allowed the recognition of nineteen races.Varying the concentration of zoospores used as inoculum, and the length of inoculation period, which did not affect the relative pathogenicities of two isolates tested, had greater effects on the level of invasion with a highly pathogenic isolate than with a slightly pathogenic one.Amongst the isolates differences uncorrelated with other characters such as pathogenicity were demonstrated in growth characters in agar culture and in abundance of zoospore production.