The control of damping-off in Pisum sativum L. by cycioheximide

Experiments on the effect of cycioheximide (Acti-dione) in controlling damping-off disease of peas confirmed that phytotoxicity would preclude its use despite its high toxicity to Pythium spp. The antibiotic did not inhibit germination of peas, but retarded growth of the shoot; the tap root was apparently unaffected, but the lateral roots were stunted.     

Factors Affecting the Production of Extracellular Pectolytic Enzymes by Phytophthora palmivora (Butl.) Butl

The nutritional requirements of Phytophthora palmivora in relationto extracellular pectolytic enzyme production were investigated.These were found to be different from those necessary for mycelialgrowth. The best carbon source for enzyme production was maltose.The best inorganic nitrogen source for enzyme production wascalcium nitrate, the effect of which could not be related solelyto the calcium ions; DL-alanine was the best organic nitrogensource for enzyme production. Evidence was obtained to indicatethat polygalacturonase (PG) and a maceration factor (MF) wereconstitutive, but that -L-arabinofuranosidase (AF) secretionwas induced by arabans present in pectin. The extrinsic factors in relation to growth and enzyme secretionwere also investigated. Both mycelial growth and enzyme productioncould be broadly related to the incubation period, temperature,and the pH of the media. Two peaks of PG and AF activities wererecorded corresponding to the phases of optimum growth and autolysisof the mycelium, respectively. When five isolates were compared,the mycelial growth could not be related to the quality of enzymesproduced.     

Biochemical studies on plant tissues infected by Phytophthora palmivora (Butl.) Butl.

Lesion formation in tomato and eggplant fruits infected by Phytophthora palmivora could be classified into three phases associated with changes in hydrolytic enzymes in the tissues. Phenolase, arabinofuranosidase and endo-polygalacturonase activities were all higher than in healthy tissue. The enzymic effects seemed to be reflected in a fairly high loss of pectic materials in the infected tissue, especially the water-insoiuble components. There was a significant negative correlation (r = -0·56) between pH of tomato and the endo-polygalacturonase secretion in the infected tissues. Paper chromatographic analysis of host metabolites showed little or no qualitative differences between the sugar contents of healthy and of infected tomato tissues; there were also no qualitative differences in the amino acid and the phenolic contents. Chlorogenic acid was detected in healthy but not in infected eggplant tissues and lesion maturation was associated with its disappearance. Gradual browning of the lesion was probably also associated with changes in cell membrane permeability of infected tissue.     

Changes in Ulrastructure following Fungal Invasion and the Possible Relevance of Extracellular Enzymes

Changes in fine structure of fruit tissues were examined byelectron microscopy for the following host-parasite relationships:Sclerotinia fructigena on apple and pear, S. sclerotiorum oncucumber, and Phytophthora palmivora on egg plant. While S.fructigena caused localized degradation of the wall region,S. sclerotiorum caused extensive wall degradation, and infectionby P. palmivora was characterized by disintegration of plasmalemmaand tonoplast. These effects appeared to be correlated respectivelywith pectolytic enzyme activity in vivo with S. fructigenaycellulolytic activity with S. sclerotiorum, and the in vitrolysis of membranes by culture filtrates of P. palmivora.