12 Harold Thurston Cook 



wet weather, both the mildew lesions and those caused by mechanical 

 injuries are soon invaded and overrun by Macrosporium. The lesions 

 then become brownish black in color. This stage is called by Edgerton 

 the "black stalk rot disease." The mildew lesions are so similar to the 

 white-spot lesions that the only reliable way of distinguishing between 

 them is by microscopical examination. 



The effect of the disease on the production of seed depends on the 

 time when infection takes place, the point at which the lesion develops, 

 and the rate of spread of the fungus throughout the plant. Early in- 

 fections usually cause greater damage than do late ones, since they 

 weaken the plant before the seeds have had an opportunity to develop, 

 and there is also more time for the fungus to spread throughout the 

 plant. However, the point of attack and the nature of the lesion formed 

 have much to do with the damage to the plant. Lesions on the lower 

 half of the stalk, especially those that girdle it, cause the stalk to fall 

 over, and the infloresence, being in contact with the moist ground, may 

 be attacked by Botrytis. Lesions that occur on the upper half and 

 rather late in the season, however, permit seed to be formed although 

 the stalk is completely girdled. Such lesions seldom cause the stalk to 

 break over, and sufficient nutrients remain in the tissues above the 

 lesions to supply the needs of the maturing seed. 



The signs and symptoms of the disease on set onions and on Egyptian, 

 or top, onions are similar to those on the seed plants and on market 

 onions. 



TIME OF OCCURRENCE 



Under New York conditions, mildew may be found in the field on 

 market onions from the middle of July to the end of the season. In 

 1926, at, Elba, New York, the first mildewed plants were discovered on 

 August 4. At that time, however, the wide distribution of the disease, 

 and the advanced symptoms on many of the plants, indicated that the 

 disease had been present for some tine. In the following year more 

 careful observations were made and the first diseased plants were found 

 on July 23, but even then it was evident that the mildew had been over- 

 looked for some time. During the next two years the results of the 

 observations were similar. Although the onion fields were inspected 

 carefully throughout the season, the first diseased plants were never 

 found earlier than July 23, but each time evidence indicated that the 

 mildew had been present for some time previous to its discovery. The 

 disease is probably present to some degree from the seedling stage. 

 Owing to the small number of diseased plants and the fact that the 

 fungus may not fruit very abundantly because of unfavorable en- 

 vironmental conditions existing early in the summer, mildew spreads 



