388 Journal of Agricultural Research 



TOXICITY OF ORGANIC ACIDS TO THE FUNGUS 



In a series of tests carried on by the writer to determine the relative 

 toxicities of the fruit acids to 5. cinerea, results were obtained with 

 regard to oxalic acid which may throw some light on the cause of these 

 differences in sporulation. Hanging-drop cultures containing large num- 

 bers of the chlamydospores in suspension in solutions of oxalic, tannic, 

 gallic, tartaric (inactive), malic, and citric acids were used. In all of 

 the tests the oxalic-acid solutions were found to be by far the most toxic. 

 As has been noted, Cooley (1914) found this acid to be produced in 

 appreciable quantities by the fungus in culture. In view of this, it is 

 very possible that in the slow development of the fungus in the resistant 

 fruits enough oxalic acid is produced by the hyphae to actually become 

 toxic to them, resulting in the production of few or no spore tufts. 



The discussion of the penetration of the fungus thus far has had special 

 reference to green and ripening plums, but not to those plums which have 

 begun to soften slightly as a result of the ripening process. It is when 

 the plums begin to soften that the fungus works the greatest havoc, and 

 it is then that variations in resistance are most noticeable in the orchard. 



Cook and Taubenhaus (191 2) were able to demonstrate a positive 

 correlation between the decrease in the oxidizing-enzym content of the 

 fruits of many plants, due both to maturing and to removal of the fruit 

 from the plant, and a decrease in their resistance to certain diseases. 

 They could show no correlation between acid content of apples and 

 pears and resistance to disease. Cooley (1914) was able to confirm 

 these latter results in the plum, finding that as the plums matured the 

 acid content increased until it reached its maximum at the time of 

 ripening of the fruit, which was also the period of greatest susceptibility 

 to the brown-rot fungus. As acidity will not explain the decrease in 

 resistance of plums to the rot on ripening, can it be explained by a 

 decrease in the oxidizing-enzym content of the plums? 



Ripe fruits of the Reagan plum, which is a resistant variety, were 

 sent to this Station from New York on October 22, 1914. On Novem- 

 ber 7 they were inoculated with the brown-rot, both by spraying on 

 spores and by laying the plums in contact with moistened mummies. 

 By this time the oxidizing enzym should have entirely disappeared, 

 owing both to ripening and to removal from the tree. In spite of this, 

 the plums were found to be still very resistant both to infection and to 

 rot after infection occurred. It is evident then that resistance can not 

 be due in this case to the presence of the oxidizing enzym. 



Material of these plums was sectioned, and it was found that in the 

 healthy tissue of these very ripe plums the middle lamella was still 



