S. R. Padwal-Desai

The involvement of phenolics and phytoalexins in resistance of potato to soft rot

SummaryPhenolics, for example chlorogenic, caffeic and ferulic acid, and phytoalexins, such as rishitin and phytuberin, were identified in potato tubers cv. Kufri Chandramukhi. The tissue of healthy tubers contained no detectable phytoalexins but did contain phenolics. The levels of these compounds were correlated with soft rot development. The rotting tissue either was free of these groups of compounds or had low concentrations. The wound periderm formed as a result of recovery from injury and infection contained high levels of the compounds. Much higher concentrations were detected at lower storage temperatures when oxygen supply was adequate. Antibacterial properties of the phenolics identified were tested againstErwinia carotovora which was inhibited by chlorogenic, caffeic and ferulic acids. The three phenolics were more effective together, in proportions in which they occurred in wound periderm, than individually. It was observed that none of these phenolics could inhibit pectolytic enzymes ofE. carotovora.

Post-harvest spoilage of mango (Mangifera indica) by Botryodiplodia theobromae

A major post-harvest pathogen of mango ( Mangifera indica ) was isolated and identified as Botryodiplodia theobromae . Its morphological characteristics, growth profile and fruit spoilage potential were studied. It formed spores endogenously within pycnidia. The organism grew well at ambient temperature (28 ± 2 °C). Citrate and phosphate buffers supported the growth of the fungus over a wide range of pH. However, acetate buffer failed to support its growth. The organism was able to proliferate on green, unripe as well as ripe fruit but needed mechanical injury for infection. The pathogen was able to cause total spoilage of the fruit within 48 h. A rise in the respiratory rate was observed when fruits were infected with B. theobromae . Fruits infected with the pathogen failed to attain a climacteric peak.

Intracellular hydrolases of Aspergillus parasiticus and Aspergillus flavus.

When the distribution profile of hydrolases in mycelial homogenates and culture filtrates of A. parasiticus and A. flavus was examined, six hydrolytic enzymes viz. N-acetyl-beta-glucosaminidase, aryl sulfatase, alkaline proteinase, cathepsin B, cathepsin D and aminopeptidase were detected in homogenate. The culture filtrates were devoid of any activity of these enzymes. The enzyme levels varied with the stage of incubation. The most abundant fungal exopeptidase showing preference for basic amino acid naphthylamides seems to be an aminopeptidase B. Incorporation of CEPA, an ethylene generating compound, stimulated the amino peptidase activity in the mycelium but inhibited the enzyme in vitro. The enzyme was also inhibited by different aflatoxins to varying degree. While aminopeptidase B was located intracellularly, a non-dialysable, heat-stable inhibitor of the enzyme was found to be secreted in the culture filtrate. This peptide inhibitor was however ineffective on the other enzymes.

Inhibition of aflatoxin-producing fungi by ethyl acetate extracts from gamma-irradiated potatoes

SummaryEthyl acetate extracts prepared from potatoes exposed to a sprout-inhibiting dose (10 krad) of gamma-irradiation were tested for inhibitory activity towardsAspergillus flavus andA. parasiticus. The treatment did not adversely affect the naturally occurring compounds which inhibit growth of these aflatoxin-producing fungi and which were still evident 4 weeks after irradiation following storage at 15°C.