416 PLANT DISEASES 



controlled by spraying with Bordeaux or lime-sulfur in very 

 early spring. 



Black-knot is a fungous disease which attacks the twigs, limbs 

 and trunks of such trees as cherry, plum, and others. Trees 

 that are systematically sprayed with lime-sulfur or Bordeaux 

 mixture for other troubles are seldom attacked by black-knot. 

 But when the disease is once established (Fig. 290) it can be 

 removed only by severe cutting away of the affected parts. The 

 wounds should be treated with lime-sulfur and then painted. 



FIELD AND LABORATORY EXERCISES 



1. Collect samples of fungi which live on dead or dying organic matter. 



2. Collect Plant Diseases and their Work. Make another collection of 

 plant diseases such as rust on blackberry leaves, rust on wheat, "cedar apple" 

 on cedar trees, and the corresponding form on apple trees, oat smut, black- 

 knot on cherry and plum trees, brown rot of peach and plum fruits and others. 



3. Study the fruit clusters in as many of these cases as possible. Explain 

 how spraying with a fungicide controls the spread of the disease. Why should 

 the spray material be applied in advance of the disease? 



QUESTIONS 



1. Give several conditions which encourage the potato scab disease, and a 



remedy for each. 



2. Describe the two kinds of blight oh potatoes. 



3. What can you say of the losses due to grain smut? 



4. Describe the effects of apple scab. 



5. What is the nature of the brown-rot disease? What fruits does it attack? 



6. What is the leaf -curl disease? 



7. Tell what you can of black-knot disease. 



8. What diseases of plants are you familiar with from your own observation? 



Make a list of them. 



References. United States Farmers' Bulletins: 15, Some Destructive 

 Potato Diseases; 219, Lessons from the Grain Rust Epidemic of 1904; 221, 

 Fungous Diseases of the Cranberry; 488, Diseases of Cabbage and Related 

 Crops and Their Control; 489, Two Dangerous Imported Plant Diseases; 

 507, The Smuts of Wheat, Oats, Barley and Corn; 544, Potato-tuber Diseases; 

 618, Leaf-spot, a Disease of the Sugar Beet; 714, Sweet Potato Diseases; 742, 

 The White Pine Blister Rust. Also 492, 555, 625, 648, 714. 



Cornell Bulletin 358, Some Important Leaf Diseases of Nursery Stock. 

 Cornell Circular 20, The Fire Blight Disease and Its Control in Nursery 

 Stock. Mass. Sta. Bui. 138, Tomato Diseases. N. J. Sta., Circ. 50, Common 

 Diseases of Beans. N. J. Sta., Circ. 51, Diseases of Grains and Forage Crops. 

 U. S. Department Bulletin 368, Brown Rot of Prunes and Cherries in the Paci- 

 fic Northwest. 



