coiild undoubtedly be prevented if the stems were continually 

 exposed to sunlight. Lettuce stem rot often attacks parsley and 

 water cress, especially when the plants grow ver}^ tall and closely 

 together, shutting out the light. Under other conditions, however, 

 the fungus is absent. Parsley set too deep in the soil will some- 

 times become affected with Sclerotinia rot, but when set out so 

 that the stems are exposed to the sunlight, it is never troubled 

 with this disease. 



The leaf blights of chrysanthemum and tomato, caused by 

 Cylindrosporium, are associated with insufficient light and circu- 

 lation of air at the base of the stem ; and lack of light is responsible 

 for many mildews. Lilac mildew, for instance, and various mil- 

 dews found on grass, the strawberry and grapevine, are more 

 common on shaded plants, as is also damping off, whether caused 

 by Botrytis or Pythium, although moisture, heat, etc. are also 

 important factors in producing this trouble. The lower leaves of 

 trellissed cucumbers and melons, which are more or less slmded, 

 become mildewed first, but such factors as moisture and vigor of 

 the foliage are also involved. Lack of light* and excessive soil 

 moisture have a bearing on the burning of foliage from the use of 

 fumigants in the greenhouse, as well as on other types of burning, 

 — top and tipburn, for example. Insufficient light has a tendency 

 to produce immature tissue which is susceptible to winter killing, 

 and spraying injuries appear to have a direct relationship with 

 light conditions. On the other hand, excess of light, combined 

 with other factors, may bring about certain troubles, but these 

 are rare and can usually be prevented in the greenhouse by care- 

 ful handling of the crop. Sun scald, for instance, is common to 

 the white pine, moose maple and many others when grown under 

 poor light conditions and suddenly exposed to bright light, and 

 some plants are so well adapted to shady locations that they can- 

 not endure direct sunlight. Asparagus, melons, cucumbers, cel- 

 ery, strawberries, ginseng and other plants are less susceptible to 

 certain diseases when grown under shelter, but this condition is 

 brought about by the absence of dews, etc., rather than of light. 

 Too much light affects transpiration materially, and the blossom 

 end rot of tomatoes is more severe when the light is bright than 

 when poor. 



There are innum.erable factors, whether single or in combina- 

 tion, that affect plants and render them more susceptible to various 

 troubles. Plants respond in various ways to stimuli, but specific 

 stimuli give rise to specific reactions, although many factors are 

 often involved in producing certain results. Too much or too 

 little soil moisture will sometimes render the plant susceptible 

 to winter killing, or in other words, corresponding results may be 

 produced by different causes. 



♦The Influence of various Light Intensities and Soil Moisture on the Growth of Cucum- 

 bers and their Susceptibility to Burning from Hydrocyanic Acid Gas. — Mass. Agr. Exp. Sta- 

 tion Rept., 1912. 



