60 



SCIENTIFIC AGRICULTURE 



October 



1920 showed the disease on the succeeding 

 crop of sunflowers in 1921, and it was en- 

 couraging to note that the disease was not 

 conspicuously worse in 1921 than in 1920, 

 despite the opportunities for spread of the 

 fungus in tlic field, the presence of infect- 

 ed debris in the field, and a damper season 

 in 1921. The losses were, however, con- 

 siderable in extent, and tlie disease is suf- 

 ficiently threatening to the sunflower crop 

 to deserve careful attention. 



The Sclerotinia lives over winter in the 

 soil, as is indicated by the fact that the 

 same areas \vere affected in 1920 and 1921. 

 Affected sunflower plants rarely produce 

 seed, and no evidence was obtained that 



•■ Figure 3. — Leisons caused by the parasite, 

 and externally produced sclerotia on sun- 

 flowers found affected in the field, August 

 19th. 1921. 



tfte seed carries the organism. Tissue cul- 

 tures were made in 1920 from the stem 

 is, 36, 24, 8, 6 and 3 inches above affect- 

 ed areas on stems and from opposite the 

 tip of a lesion wliich extended further up 

 one side of the stem than the other. In 

 none of tliese cases was Sclerotinia myce- 

 lium obtained. The organism may, how- 

 ever, be obtained readily from tissue cul- 

 tures from affected areas. It is evidently 

 not systemic in affected plants. 



Tnoculations were made during the win- 



ter of 1920- '21 upon sunflower and other 

 plants in the greenhouse, and upon various 

 vegetables. Cultivated sunflower plants 

 about one foot high were killed by inocula- 

 tion of the base of the plants with mycelium 

 from a pure culture of the fungus and 

 Sclerotia developed within the stem. The 

 lack of light allowed only a slight growth 

 of the plants. In the laboratory inocula- 

 tions, a culture of the fungiis into carrots 

 and parsnips readily produced rot, and a 

 considerable rot was produced upon potato 

 tubers. Onions rotted onlj' slightly. Tur- 

 nips and beets did not rot in the preli- 

 minary tests made. Corn plants were not 

 affected when the fungus was inoculated 

 into the base of the plants in the green- 

 house. 



Tests were made during the summer of 

 1921 relative to the rate and manner of 

 spread of the disease in the field. An 

 isolated plot of sunflowers was planted, 

 and on July 26th, 1921, inoculations were 

 made by placing mycelium from an af- 

 fected plant against needle punctures at 

 the base of two plants; then the diseased 

 stalk was placed between iwo uninjured 

 plants and merely pressed into the surface 

 of the soil. In one week's time, both 

 punctured plants showed the disease cons- 

 picuou.sly, and the plants on either side of 

 the diseased stem, also were blackened 

 at the base by the attack of the 

 fungus. The spread continued to ad- 

 jacent plants; on Aug. 18th, 5 plants 

 along the row in one case and 11 plants in 

 the other case in which punctures were 

 made, were diseased. The fungus had 

 spread along the row and caused rot for a 

 distance of 2i/^ feet in 22 days in the case 

 in which 11 plants were affected. No 

 I)lants in the plot, other than these men- 

 tioned, became affected. No injuries ex- 

 cept those that may naturally occur are 

 necessary for infection. 



Tests were not made with varieties • of 

 sunflower to determine whether resistance 

 to the disease is to be found. The fact 

 that so many hosts, including wihl sun- 

 flowers and other weeds, may be {dtacked, 

 makes the |>rospect for resistance, especial- 

 ly where the soil is heavily infested with 

 the fungus, not very bright. Healthy 

 plants are sometimes found in areas in the 

 field in wliich practically every i^lant.is 



