Constituents of orange juice that stimulate the germination of conidia of Penicillium digitatum.

PELSER, P. du T. , and J. W. ECKERT. 1977. Constituents of orange juice that stimulate the germination of conidia of Penicillium digitatum. Phytopathology 67: 747-754. Conidia of Penicillium digitatum germinated 90% in a myo-inositol was inactive. A solution containing 76.8 mg diluted dialysate of orange juice, but not in distilled water. glucose and 29.4 mg citric acid/ 100 ml, concentrations Maximum germination was observed in 1% (v/v) orange equivalent to 3% orange juice dialysate, resulted in 31% juice dialysate at pH 3.5 -8.0, but germ tube development was germination and more vigorous germ tube development than most vigorous at pH 4.0 5.5. The constituents of the that observed in solutions containing higher concentrations dialysate which were adsorbed by an anion exchange resin of glucose alone. Several other organic acids likewise were (anionic fraction) were most active in stimulating stimulatory when combined with glucose, but succinic, germination, whereas the cationic and neutral fractions fumaric, maleic, and acetic acids were inactive. The supported vigorous germ tube development, but did not combination of glucose and citric acid produced higher initiate germination. A mixture of L-ascorbic, citric, and Lgermination at pH 4.5 than at pH 6.0. Relatively simple malic acids stimulated germination to essentially the same mixtures of nutrients stimulated a high rate of germination. level as a dilution of the anionic fraction which contained the Maximum germination was observed in spores incubated in same concentration of these acids. Citric or L-malic acids, (i) glucose combined with one of several organic acids, (ii) alone or combined, did not significantly increase glucose combined with L-ascorbic acid, and (iii) citric acid germination. Both L-ascorbic and dehydroascorbic acid combined with L-ascorbic acid. Exogenous nitrogeninduced a higher level of spore germination than isoascorbic containing nutrients were not required for maximum acid (D-ascorbic acid). Of the major carbohydrates present in germination, but vigorous germ tube growth was observed the neutral fraction of orange juice, glucose (2.56 g/ 100 ml) only in solutions containing mixtures of amino acids and alone supported 50-70% germination, whereas equivalent vitamins. concentrations of fructose and sucrose were less effective and Additional key words: nutritional requirements for spore germination. Penicillium digitaturn Sacc. is the major cause of formulate a completely synthetic medium which would postharvest decay (green mold) of citrus fruits produced possess activity equivalent to orange juice in this respect. in arid subtropical regions. The fungus has not been reported to be pathogenic on other fruits. Conidia of P. MATERIALS AND METHODS digitatum are abundant in the atmosphere of citrus groves and packinghouses throughout the world and fruit All chemicals were the purest available from infection is initiated by germination of spores in moist commercial sources and the water was distilled twice; the injuries in the peel of the fruit. Several investigators have second time in a glass apparatus. Conidia of P. digitatum demonstrated that the incidence of infection may be (California isolate M6R of this laboratory) were increased significantly by the addition of certain nutrients harvested from decaying lemons which had been to spore suspensions used for inoculation of fruits (4, 18, inoculated aseptically with a pure culture of the fungus. 19). Penicillium digitatum has been cultivated on The conidia were brushed from the surface of the lemons. synthetic media, but mycelial growth is increased passed through a 7 -4m sieve to remove any fruit debris substantially by the addition of complex mixtures of and water droplets, and stored in a glass vial over nutrients to the minimal medium (14, 25, 26, 29). Rapid anhydrous CaSO 4 (Drierite®) at room temperature (22and complete germination of spores has been observed 24 C). Dry conidia were suspended in sterile 0.05% (v/ v) only in media containing extracts of citrus fruits or other Tergitol XD (polyalkylene glycol ether, Union Carbide materials (10, 19, 25), but no systematic study of the Chemicals Co., NY 10017) with the aid of a glass tissue nutritional factors required for germination has been homogenizer and the suspension was adjusted to contain reported. approximately 106 conidia/ ml. One ml of conidial The present investigation was undertaken to identify suspension was added to 24 ml of nutrient medium after the components of orange juice which are required for pH adjustment with dilute KOH or HCI. Triplicate 0.2 ml vigorous development of conidia of P. digitatum and to portions of the seeded nutrient solutions were placed in glass microbeakers (5 mm deep, 10 mm in diameter) and Copyright © 1977 The American Phytopathological Society, 3340 were incubated in a water-saturated atmosphere at 25 C Pilot Knob Road, St. Paul, MN 55121. All rights reserved, for 16 hr. One hundred conidia in each of the triplicate