390 PHYSIOLOGY OF THE FUNGI 



That resistance is due to the presence of some toxic substance, perhaps 

 an organic acid or some related compound, in the living host cell is one 

 of the most popular theories. However, detailed proof of the effective- 

 ness of such a compound, even though present in the plant, is often diffi- 

 cult to obtain. Cook and Taubenhaus (1911) list some organic acids in 

 order of their toxicity as follows: tannic, gallic, malic, tartaric, and citric. 

 They state that vegetable juices contain an enzyme which acts upon 

 gallic acid to produce tannin or a tannin-like compound which is toxic 

 to fungi. The amount of the enzyme decreases with maturity and 

 ripening of the fruits (apples, pears, persimmons, etc.), which accordingly 

 become more susceptible to rot fungi. 



An outstanding example of chemical resistance is that described by 

 Link and Walker (1933) for onion smudge caused by CoUetotrichum 

 circinans. The cell sap of the colored varieties (resistant) is much more 

 toxic to the fungus than the cell sap of the white-skinned varieties (sus- 

 ceptible) . Furthermore, the sap of the colored varieties contains catechol 

 and protocatechuic acid in amounts that would account for the resistance 

 of these varieties to the fungus. The action of volatile and nonvolatile 

 antibiotics in the fleshy scales of the onion is believed to be a definite 

 factor in relative resistance of onion varieties to C. circinans, Aspergillus 

 niger, and Botrytis allii (Hatfield et al., 1948). Reynolds (1931) explains 

 resistance of flax varieties to Fusarium lini as being due to the higher 

 amounts of glucosides, which upon hydrolysis yield hydrocyanic acid. 

 Similarly, the resistance of species of Solamim to Cladosporiuni fulvum 

 is believed to be due to the presence of higher amounts of solanine 

 (Schmidt, 1933; cited by Brown, 1936). Rochlin (1933) believes that 

 there is a direct connection betw^een resistance of crucifers to clubroot 

 and the amount of glucosides, which on fermentation give rise to pungent 

 mustard oils. The isolation of 2-methoxy-l,4-naphthoquinone from 

 Impatiens balsamina was reported by Little et al. (1948). This substance 

 had a high antibiotic activity against several fungi and was nontoxic to 

 tomato and bean plants. This may be an example of a naturally occur- 

 ring antibiotic as a factor in resistance. Fontaine et al. (1947) suggest 

 that tomatin may be a factor in the resistance of certain tomato varieties 

 to Fusarium lycopersici. 



An interesting theory of resistance to obligate parasites is presented 

 by Dufrenoy (1936). He divides the hosts into three groups: (1) highly 

 resistant, (2) moderately susceptible, (3) extremely susceptible. He 

 believes that, when a fungus enters the highly resistant host, it kills the 

 cells it penetrates and that the death of these cells alters the metabolism 

 of the surrounding cells, so that their cell sap becomes rich in phenolic 

 compounds, which prevents the further invasion by the pathogen. In 

 the moderately susceptible host the host cells and their living contents 



