SYMPTOMS OF DISEASE IN PLANTS. 235 



Leaves affected by this trouble show large, irregular blotches of black- 

 ened tissue, in fact they look almost as if they had been spattered with 

 drops of tar. When these spots become abundant the intervening tissue 

 of the leaf turns yellow, and the leaves are dropped quite early. In 

 severe epidemics considerable injury may result from these "tar-spots." 

 In all of our cereal rusts, the winter stage is indicated by either small, 

 black-covered pustules or by elongated, black streaks, having a some- 

 what pov/dery appearance. The winter spore pustules of the orange leaf 

 rust of wheat are small and covered and are generally confined to either 

 leaf blade or sheath, while the winter spore pustules in the stem rust 

 of wheat, rye or oats are in the form of elongated, naked streaks, which 

 are more numerous on the stem than on any other part of the plant. 

 In a leaf disease of the honey locust, the leaflets often show numerous 

 small black spots on the under side; in many cases these black spots 

 become so numerous that they coalesce and cover the entire surface 

 of the leaflet, hence the common name "black leaf" disease. Another 

 fungus trouble that is quite common in Nebraska orchards is the so- 

 called "fly-speck" fungus of the apple. Fruits attacked by this fungus 

 show small clusters of circular black spots which on account of their 

 characteristic appearance have suggested the common name. This fungus 

 is more common and evident on the yellow-skinned varieties. The 

 "sooty fungus" produces larger, and more diffuse, smoky brown, or al- 

 most black, irregular patches upon the epidermis of apples. 



Leaves which show circular or sometimes irregular perforations have 

 probably been affected by some fungus parasite. In many cases leaves 

 so affected will show brown spots of dead tissue which have not yet 

 dropped out. Several diseases have been called "shot-hole" diseases on 

 account of this peculiar effect upon the foliage. Notable among these 

 troubles is the shot-hole disease of the cherry and plum. This disease 

 is sometimes called "rust" by orchardists, but the true cherry rust is 

 an entirely different disease. Nearly all of the English Morello cherry 

 trees in the eastern half of Nebraska have been killed by this trouble 

 during the past few years, while other varieties have suffered to a 

 considerable extent. A "shot-hole" disease of the peach has been espe- 

 cially abundant during the past season in Nebraska. Unless its spread is 

 checked by natural means or by spraying, it bids fair to cause extensive 

 losses in the peach orchards of the state. 



A sudden wilting of a plant which cannot be explained by dry con- 

 ditions of soil or air is often due to the attack of a parasite. Both 

 seedlings and mature plants exhibit this symptom, and the loss which 

 is occasioned by diseases having this attendant symptom is often enor- 

 mous. In a seedling the disease in which this symptom is prominent 

 is often called "damping off." The young plant drops dead as it were, 

 for a fungus parasite has entered the stem at about the ground level 

 and has struck a vital blow, interrupting essential physiological func- 

 tions. A disease of the squash and related forms called "wilt" is not 



