278 Journal of Agricultural Research voi. xiii, no. s 



Such diseased plants should not be thrown on the manure or compost 

 heap. No doubt the best method of disposing of them is to bum them. 



It can not be said that this collection of diseased material can be 

 relied upon to do more than remove the gross sources of infection. It is, 

 furthermore, a preventive measure which is extremely effective against 

 the other lettuce diseases, notably, lettuce-drop and lettuce-graymold. 



This disease is commonly found where the splashing or the dripping 

 of water occurs. The repair of gutters or of valves is of obvious neces- 

 sity. Few growers realize fully the extent of damage which comes 

 from this source. It is not uncommon to see a 10 per cent loss due to 

 the dripping of water the full length of a house. The sprinkling of 

 plants with the hose is an inefficient way of watering. It is rarely 

 necessary to rinse off the leaves of plants in beds, and when hose water- 

 ing is necessary, the water can be safely and quickly applied by flowing 

 the stream along the ground. Where soil conditions permit, subirriga- 

 tion is an excellent method. 



The most popular method of greenhouse watering is by some type 

 of overhead system. It might seem at first glance that this method 

 would have the same disadvantage as sprinkling with a hose. It has 

 been found by observation that in the overhead systems the splashing 

 from plant to plant, or even from leaf to leaf, does not occur. The jets 

 of water are too fine and the force too slight when the water reaches 

 the plant to bring about any splashing. The writer has no knowledge 

 of an outbreak of lettuce anthracnose in greenhouses watered by the over- 

 head system. The only diseased plants found with such watering 

 systems have always been associated with leaky valves or leaky gutters. 

 The glutinous nature of the spores of the anthracnose fungus explains 

 why these bodies can not be transferred from plants except when the 

 diseased spot is soaked with water. In the field, cultivation should be 

 avoided when the plants are wet. The transference of the disease in 

 the greenhouse is almost wholly brought about by spreading of water. 

 The experiment reported and the diagrams (fig. 2) show most conclusively 

 the relation of this factor. 



The recommendation, therefore, is to do away with splashing of water 

 from plant to plant or leaf to leaf. The grower may accomplish this in 

 several ways. The overhead systems are efficient, and are undoubtedly 

 the most popular method of greenhouse watering. 



There is possible also the adjustment of the growing conditions so as 

 to favor the lettuce plant and to check the fungus. The most efficient 

 agent in this regard is ventilation. The spores of the fungus will probably 

 not germinate and enter into the lettuce leaf when transferred, unless the 

 leaves stay wet for a considerable time — 6 to 1 2 hours. Since ventilation 

 can not wholly compensate for heavy watering, and as humid conditions 

 will permit germination and penetration of the leaves, excessive watering 

 should be avoided. The most successful lettuce growers aim to keep the 



