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THE MILDEW SPHAEROTHECA CASTAGNl ON BIDENS 

 AS A FUNGUS FOR CLASS USE 



Illo Hein 



In the course of my studies on the mildews I have found that 

 Bidens frondosa lends itself admirably to the growing of Sphae- 

 rotheca castagnei on the leaves of potted plants in the greenhouse 

 Bidens is very common in fields, waste places and roadsides, and 

 in the late summer and early fall the mildew spreads abundantly 

 all over the plant; it is, in fact, difficult to find plants at this 

 time that do not have some fungus on them and most plants are 

 literally covered with it. Towards the end of September 

 infected Bidens were brought into the greenhouse, potted and 

 kept growing until plants grown from seed were ready for 

 infection. The first sowing was made October 24, and when 

 the seedlings were four weeks old they were placed among the 

 infected field plants. It was soon found that the mildew spread 

 naturally from plant to plant and a week after exposure a few 

 ripe perithecia were visible on the leaves. A second sowing was 

 made December 4, the plants producing perithecia after the 

 holidays. No difficulty was found in transferring the mildew 

 from one host plant to another; it spread naturally from field 

 plants to greenhouse plants. That the spores are carried a 

 considerable distance by light currents of air in the greenhouse 

 is shown by the fact that uninfected plants placed over ten 

 feet away from the infected plants showed abundant growth of 

 mildew about as soon as the plants that were growing close by. 

 To insure infection an infected plant should be placed in the 

 midst of the new plants, but this precaution is perhaps unneces- 

 sary. Successive crops of Sphaerotheca have been grown in 

 the greenhouse throughout the winter, the fungus spreading 

 with undiminished vigor. 



A point of interest is that the host plant during the period 

 of shortened daylight comes to maturity very early. The plants 

 grown from seed sown in December started to produce flowers 

 after four weeks and in five weeks, though they were but seven 

 inches tall, many plants had ripe seeds and then died off rapidly. 

 This early maturing can be delayed somewhat as has been 

 shown in the case of other lots of plants, by picking ofi'" the buds 

 as soon as they appear. 



