George A. Zentmyer

Chemotaxis of Zoospores for Root Exudates

A chemotactic response of the zoospores of a soil-inhabiting plant pathogenic fungus, Phytophthora cinnamomi, for roots of avocado seedlings was observed. The chemotaxis of the zoospores and chemotropy of their germ tubes were directly related to infection and disease production. Indications were obtained of specificity of the pathogen-attracting root exudate, and interesting implications are evident with regard to mechanisms of invasion and pathogenicity, and to disease resistance.

Origin and distribution of four species of Phytophthora

Information is presented on possible origins, and on the distribution of four species of Phytophthora: P. infestons, P. cinnamomi, P. palmivora and P. colocasiae . The origin of P. infestons appears to be Mexico where both mating types appear on native species of Solanum , where resistance to the pathogen occurs, and where high variability is found in the population of the fungus. The A1 mating type of this pathogen is the form that has been distributed widely around the world. Recent discovery of the A2 type (previously believed confined to Mexico), in the British Isles, Egypt, and Switzerland, raises some interesting questions and the situation needs further elaboration. P. infestans has undoubtedly been spread around the world by man, probably largely in the potato tuber, shipped to various countries as food or new sources of seed. Data on the centre of origin of P. cinnamomi are more confusing, but there are indications of an Asian origin ranging from New Guinea possibly through Indonesia, Sumatra and Malaysia and including Taiwan, and also another centre in the southern Cape Province of South Africa. The A2 mating type of P. cinnamomi is very widely distributed, with the A1 much less common but predominant in New Guinea and southern South Africa. It could have been carried from an Asian area of origin into Latin America or other areas on soil and plant material transported in voyages by Spanish and other explorers, possibly beginning in the seventeenth century and continuing with increased commerce across the Pacific Ocean. Undoubtedly P. palmivora has been distributed around the world primarily by man, from a possible origin in Central or South America. There are several native hosts in that area, including cacao, rubber, avocado, pineapple, papaya and cotton. Several resistant species are known on which the fungus may have had its origin. Both mating types of P. palmivora are found in many areas around the world, although the A2 is somewhat more commonly isolated. Little information is available on the origin of P. colocasiae , but there are indications of an Asiatic origin. The fungus has undoubtedly been distributed by means of vegetatively propagated material, and probably by soil.

An Improved Synthetic Medium for Oospore Production and Germination of Several Phytophthora Species

Growth of Phytophthora arecae, P. cactorum, P. cambivora, P. capsici, P. cinnamomi, P. citricola, P. citrophthora, P. colocasiae, P. cryptogea, P. drechsleri, P. erythroseptica, P. fragariae, P. heveae, P. lateralis, P. meadii, P. megasperma, P. megasperma var. sojae, P. palmivora, P. parasitica, and P. syringae on a chemically defined medium was comparable to that on a V-8 juice agar medium. A majority of the species produced oospores on the synthetic medium. Oospores were also produced by the interspecific hybrids, P. capsici (Al) X P. parasitica (A2), P. capsici (Al) x P. palmivora (A2), P. cinnamomi (A1) X P. cambivora (A2), P. cinnamomi (A1) X P. drechsleri (A2), and P. parasitica (A1) X P. citrophthora (A2). Germination of oospores obtained from the synthetic medium was comparable to, or better than that observed for oospores produced on V-8 juice agar medium.

Bacterial Stimulation of Sporangium Production in Phytophthora cinnamomi

Bacteria, notably Chromobacterium violaceum, stimulate initiation of production of sporangia by Phytophthora cinnamomi, a plant pathogen which does not produce this asexual stage in ordinary agar or liquid culture.

Mechanism of zoospore release inPhytophthora andPythium

Abstract The flow characteristics during zoospore release and cytoplasmic discharge from sporangia were studied in double-distilled water using severalPhytophthora species andPythium aphanidermatum. The time for passage of a zoospore through the exit pore increased and flow velocity decreased linearly with increasing volume of discharged cytoplasm fromPhytophthora sporangia (three-phase germination type) as well as fromPythium sporangia. The number of zoospores per sporangium averaged 31 (P. cactorum), 38 (P. palmivora), and 68 (P. cinnamoni); the mean passage time per zoospore was about 1.40, 0.90, and 0.50 seconds, and the mean flow velocity was about 13,20, and 29 μm per second for the three species, respectively. Sporangia produced in bean meal broth released zoospores with a higher velocity than sporangia produced in a chloride salt solution. The different flow rates could be correlated with the diameters of the exit pores of the sporangia and with turgor pressures at the beginning of cytoplasmic discharge. Zoospore release fromPhytophthora sporangia could be stopped completely by transferring sporangia into solutions of osmotic water potentials between −2.4 and −3.6 bar. Cytoplasmic discharge is explained using an osmotic pressure-flow model based on the equation of Hagen-Poiseuille.

Origin and significance of the discharge vesicle in Phytophthora

During zoospore release from sporangia, all Phytophthora species studied developed a discharge (germination) vesicle into which 30 to 70% of the total number of zoospores were released before the vesicle burst. Vesicles of P. cactorum, P. Palmivora , and P. cinnamomi reached maximal diameters of 26, 40, and 46 μ m, respectively, when sporangia were placed in double-distilled water for zoospore release. The mean velocities of surface area enlargement of the vesicles represent ratios of 1:1.7:4.6, comparing P. cactorum with P. palmivora and with P. cinnamomi . For sporangia of P. cactorum , the surface area of the vesicle enlarged at a constant rate, whereas the increase in vesicle volume proceeded with an acceleration of about 79 μ m 3 /second 2 . When sporangia produced by P. cactorum grown in either bean meal broth or a chloride mineral solution were transferred to double-distilled water they developed discharge vesicles that enlarged in surface area at velocities of 224 and 145 μ m 2 /second, respectively, and had turgor pressures of about −8.0 and −4.8 bar, respectively. During zoospore release in P. palmivora , the outer sporangial wall and an amorphous cover layer were ruptured, and the papillar material (vesiculogen) was forced through the exit pore to form a continuous and plastic vesicle wall consisting of thin fibrillar elements of undetermined lengths (EM). The opening of the sporangium of P. palmivora , the enlargement of the vesicle, and subsequent zoospore release could be retarded or stopped in any stage by transferring germinating sporangia to CaCl 2 or PEG solutions with osmotic potentials between −2.5 and −3.5 bar. During plasmolytic treatments, the papillae of sporangia of P. cactorum and P. palmivora kept their original shape, whereas the apices of P. cinnamoni sporangia collapsed.

Oospore Morphology and Germination in the Phytophthora Palmivora Complex from Cacao

Oospore morphology and germination were studied using 17 isolates representing three morphological forms (MF) of Phytophthora palmivora from cacao. Oospore production was most abundant on carrot ag...

The Influence of Temperature and Nutrition on Formation of Sexual Structures by Phytophthora Cinnamomi

The influence of temperature on the abundance, developmental sequence, morphology, and size of Phytophthora cinnamomi sexual structures was studied in a cross between isolates of A1 and A2 mating types, and in stimulation by avocado-root extract of homothallic oospore production in single cultures of an A2 isolate. Gametangia were formed from 9 to 33 C, and oospores from 12 to 30 C. The optimum temperatures for oospore production were 15 to 21 C in the cross and 18 to 24 C for homothallic production in root extract. Temperature did not significantly influence size of sexual structures, but did affect the rate of development and abundance. Nutrition greatly influenced the sexual stage: significant differences in size and abundance of sexual structures were observed using the same cross on different media. Oogonia and oospores produced homothallically in root extract and those from the cross on a synthetic medium were larger than those from the cross on four natural media. Comparisons of sexual-structure measurements for taxonomic purposes should be made between structures produced on the same medium.

EFFECT OF PHYSICAL FACTORS, HOST RESISTANCE AND FUNGICIDES ON ROOT INFECTION AT THE SOIL-ROOT INTERFACE

Summary Root rots incited by species of Phytophthora provide unique systems for examining factors of the soil involved in pathogenesis. Sporangium production is influenced by the microbial population, by the osmotic and matric potentials, by soil temperature and other soil factors. Chemotaxis of zoospores, the major mechanism for attraction to roots, is followed by invasion by the large numbers of germinating zoospores, and then by root infection. Infection, but not necessarily Chemotaxis, can be influenced by host resistance, and by organic fungicides applied either to soil or as systemics to the foliage. In species of Persea resistant to P. cinnamomi , Chemotaxis may be active but root invasion is limited. A preformed toxicant, borbonol, is involved. Two new organic fungicides, the acyl alanines (CIBA-Geigy), and organic phosphites (aluminum ethyl phosphite, Rhone-Poulenc) greatly reduce root invasion and infection when applied to the soil, or when applied to the foliage, the latter indicating downward translocation. The acyl alanines drastically reduce growth and sporangium production of the pathogen in vitro , and probably similarly in soil. The phosphites have little influence on Phytophthora in vitro but are equally effective to the acyl alanines in vivo. Resistance apparently affects invasion more than the interactions at the soil-root interface. The systemic fungicides may affect both Chemotaxis and invasion. Moisture potential (matric and osmotic) affects the pathogen by favouring or reducing sporangium production by P. cinnamomi . This obviously affects zoospore production, Chemotaxis, and invasion; and variously influences other components of the microbial population. Thus there is a complex system of interactions in relation to root diseases that is variously influenced by biological, physical, and chemical factors. Identification of the precise relationships will provide valuable information on disease development and control.

Comparisons of single-oospore isolates of Phytophthora species from naturally infected cocoa pods in Brazil

Soil, mummified cocoa pods on trees, and pods from husk piles were examined for species of Phytophthora in several experimental cocoa field plots in Brazil. Phytophthora capsici and P. citrophthora were recovered from soil and from husk piles. Compatibility types Al and A2 of P. capsici were isolated from a single pod. Plerotic oospores of Phytophthora with amphigynous antheridia still attached to the oogonial stalk were found in pods from husk piles. Fifteen percent ofthe oospores removed from pods germinated. Twenty single-oospore isolates were compared with respect to colony and sporangia morphology, sporangium production, stalk lengths, and compatibility type. Most cultures from ger? minated oospores resembled P. capsici but some had unique characteristics that included noncaducous sporangia and large (> 50 /nm in length) sporangia. The majority of single-zoospore isolates from singleoospore cultures were similar to their respective parent cultures. However, some differed with respect to size of sporangia, caducity of sporangia, and compatibility type.