264 DISEASES OF CULTIVATED PLANTS 



above account it will be seen that excess of moisture in the 

 soil and in the air is the primary cause of the disease, the 

 fungus being only a secondary agent, although responsible for 

 the greatest amount of injury; but for an imperfect mode of 

 cultivation it could not have proved injurious. It will pro- 

 bably be argued that a considerable amount of moisture 

 constantly present in the soil is a necessity under the condi- 

 tions ; this may be so, nevertheless it is the primary cause of 

 the trouble. Lettuce grown out of doors is not subject to 

 this disease. 



Good ventilation, also good drainage to prevent a sodden 

 condition of the soil, are of primary importance. Spraying 

 with potassium sulphide, which can only be done while the 

 plants are young, will check the rapid spread of the disease ; 

 in fact, even if no disease is present, spraying, so as to wet the 

 surface of the soil, will do much to prevent infection. Where 

 disease has existed it may be taken for granted that the soil is 

 infected, and should be sterilised before another crop is sown. 



Bulb sclerotinia (Scltrotinia bulborum, Rehm.) often proves 

 very destructive to bulbs, including those of the onion, 

 hyacinth, Stilla, Crocus, etc. The first indication of the 

 disease is the presence of yellow stripes and blotches on the 

 leaves during late spring or early summer. If the weather is 

 damp and cloudy these yellow spots soon produce a crop of 

 minutely velvety, olive-brown mould ; this is the conidial or 

 Botrytis form of the fungus, the spores of which are produced 

 in quick succession during the summer, and are washed by 

 rain or carried by wind to adjoining plants, which are in turn 

 infected. The mycelium passes down the leaves and stem 

 into the bulb, where it forms small black sclerotia in the 

 substance of the bulb-scales, more especially the outer ones. 

 During the following spring ascophores on slender stems 

 sometimes spring from the sclerotia. At other times the 

 sclerotia bear the Botrytis form of fruit. 



Ascophore brown, 8-12 mm. across, stem elongated, slender; 

 spores 14-16x7-8 /*. Spores of Botrytis form elliptical, 

 hyaline, 9-10 X 7 /*. 



When the yellow patches are first seen on the leaves, spray 

 with potassium sulphide ; it is important to spray uninfected 

 plants also ; this checks the spread of the epidemic, but does 

 not prevent the mycelium from passing into the bulbs 

 of plants already infected. Badly infected bulbs, having 



