Fuether Studies on the Relation of Onion Scale Pigmentation to Disease Resistance »

In earlier studies of the nature of parasitism and resistance associated with the smudge disease {Colletotrichum circinans [Berk.] Vogl.) of onion bulbs (3, 4) it has been shown that the host tissue contains certain substances which are toxic to the invading organism. These substances fall into two classes, (a) the volatile oils and certain associated nonvolatile compounds in the cell sap (6) and (b) the toxins associated with scale pigmentation. The volatile substances which arise from the succulent tissue, especially upon wounding, are readily taken up by liquid suspensions of conidia and when present in sufficient concentration completely inhibit germination and retard growth. No marked consistent difference in the amount of these volatile toxins given off by different varieties of onion has been noted, and the conspicuous resistance of colored-bulb varieties as compared with the white types is not to be explained on this basis. The volatile substances, while generally toxic to fungi, apparently are not so effective upon rapid bulb-decaying organisms, such as Botrytis allii and Fusarium cepae, as upon milder parasites, such as Colletotrichum circinans. ' It has been suggested therefore that these toxins in the juice of succulent onion scales may have a rôle in restricting the advance of certain invading organisms (6). ' The second group of toxic substances was found in the water "extract from the dry outer colored-bulb scales from which the volatile substances had disappeared (4). Spores placed in extracts of sufficient concentration were either entirely prevented from germinating or, if germination started, the process was so modified as to cause the exudation of naked cytoplasm from the tip of the young germ tube, thus inactivating the organism. As the concentration of the extract was reduced, the toxic effects were proportionately decreased until eventually normal germination and growth occurred. In contrast to this, extracts from white scales showed little or no toxicity and supported good germination and growth. It appeared therefore that the natural resistance of colored onion bulbs to the smudge organism is based upon the toxic substances associated with the pigmentation of scales. It is presumed that in nature amounts of these toxins sufficient to inactivate the fungus (if present) dissolve into drops of meteoric or soil water which ^ come in contact with colored scales. The outer colored scales therefore serve as a barrier to invasion by the parasite. In the course of this investigation it became of interest to extend the same line of experimentation to a number of other fungi, especially those which are parasitic upon onion bulbs. The relation of onion oils to a number of organisms has been reported (6). The present paper deals with the relation of the pigment extracts to practically the same groups of fungi, namely, Fusarium cepae Hanzawa, Fusarium sp. No. 45 (an onion bulb-rotting organism) , Fusarium lycopersici Sacc. (tomato wilt organism), and Fusarium graminearum Schwabe (Gibberella saubinetii (Mont.) Sacc.) (wheat scab organism); onion neckrot fungi, Botrytis allii Munn, Botrytis sp. No. 110, and Botrytis sp. No. 108a; Aspergillus sp. No. 4660 3 and Aspergillus nig er van Tiegh.; and an unidentified species of