36 U. S. DEPARTMENT OF AGRICULTURE. 



of the leaves throughout the head. It is found in head let- 

 tuce from all except the Boston district. 



Tip burn is a field disease probably caused by an irregular 

 water supply. The injury is most likely to occur when 

 bright warm weather follows a period of cloudy or rainy 

 weather, and is much worse on certain soils than on others. 

 The disease seems to be least prevalent on a soil with a high 

 sand and low clay content, combined with a high water- 

 holding capacity. In such a soil, lettuce becomes deep 

 rooted, and can apparently obtain water with sufficient 

 rapidity to prevent injury from too rapid transpiration. In 

 the Imperial Valley of California, this disease appears to be 

 associated with an excess of alkali in the soil. 



Tip burn often predisposes affected stock to slimy soft rot 

 during transit and storage. The slimy soft rot starts in the 

 dead tissues, especially in the interior of the head where 

 moist conditions prevail. Sometimes the rot gets a foothold 

 even in the growing plant. 



No effective method of control has been established for 

 tip burn. A well-regulated water supply and the use of 

 varieties properly adapted to the soil and climatic condi- 

 tions are important considerations. 



Ref. (19); (20). 



LETTUCE: MISCELLANEOUS DISEASES. 

 DOWNY MILDEW. 



Cause: A fungus (Bremia lactucae). 



The symptoms of this disease are sharply angular leaf 

 spots, yellowish to brown when viewed from above, and 

 bearing on the lower surfaces a white felt-like outgrowth of 

 mold, the spores and spore-bearing mycelium of the fungus. 



Downy mildew occurs on lettuce grown under glass and 

 on the field crop where moist cool weather prevails, as in 

 the Colma district in California. In head lettuce, the inner 

 as well as outer leaves are attacked. This disease is of some 

 importance because of its direct attack on the edible leaves 

 and because of its tendency to predispose the tissues to slimy 

 soft rot. 



LEAF SPOT. 



Cause : A fungus (Septoria consimilis) . 



This disease causes brown spots on the older leaves bear- 

 ing black points, the pycnidia of the fungus. It occurs on 

 garden varieties late in the season, and is of little impor- 

 tance. 



LEAF SPOT. 



Cause: A fungus (Cercospora lactucae). 



Small tan-colored spots on the older leaves are the symp- 

 toms of this disease, which has been noted in the San Fran- 

 cisco market. 



SHOT HOLE OR ANTHRACNOSE. 



Cause: A fungus (Marssonia panattoniana). 



