62 NEBRASKA STATE IIOKTICULTIKAL SOCIETY 



tend deeper than the bark, while the blister canker attacks both bark and 

 heart wood and is the most difficult to control. Because of the fact that 

 the last named disease is much more prevalent than the others, is much 

 more destructive, and that the remedies recommended for ii apply equally 

 well to either of the others, only Illinois or blister canker will be dis- 

 cussed. 



This canker is easily recognized, even by a casual observer. It does 

 not require close observation to distinguish dead branches of trees that 

 are killed by this fungus. The flow of sap is cut off and the bark dies. 

 Often a branch is killed in a single season and the leaves and apples 

 cling to the twigs for some time. Often the apples liang on all winter. 

 At the base of the limb and often over its entire length may be seen at 

 first blister-like protrusions. As the disease progresses, the tops of these 

 protrusions become more flattened and more nearly resemble nail heads. 

 If a thin layer is cut away with a knife, irregular circular spots of tan 

 surrounded by a dark brown or black ring may be seen. These spots vary 

 in size from 1-16 to 1-4 inch and are scatterec' all over the diseased sur- 

 face in most cases, though sometimes few if any are present. Two kinds 

 of spores are produced from these spots which spread tlie disease. Tlie 

 first set of spores (conidia) are produced in the months of July, August 

 and September. Masses of mycelium form in more or less compact 

 bunches called stroma; from these masses slender stalks arise, sometimes 

 singly and simple, sometimes branched, on the ends of which the spores 

 are formed. This is what causes the blister-like appearance on infected 

 bark. The spores are set free by the cracking of the epidermis covering 

 these masses and are carried by tlie wind from tree to tree, producing 

 canker wherever the spores lodge in wounds, If conditions are favorable. 

 The following season during the months of April, May, and June another 

 kind of spores are produced from these same stromatic masses. Unlike 

 those of the first season they are formed in sacks (asci), each sack con- 

 taining eight spores. The sacks are also enclosed, several together in 

 cases (perithecia) which are arranged in a layer throughout the irregular 

 masses. Closing the opening of each perithecium is a plug which pre- 

 vents the escape of the spores, until it has been sloughed off or disinte- 

 grated by the action of the weather or removed by some mechanical 

 means, such as being cut off by pruning tools, the rubbing of limbs to- 

 gether, etc. When freed, the spores cause infection in the same way as 

 the condition of the season before. 



Infection may arise through spores lodging either in wounds in the 

 bark, or where branches have been removed by pruning. The cankered 

 trees at the Nebraska station and many orchards over the state were 

 examined, and 80 per cent of the cankers were found surrounding wourrds 

 caused by pruning and broken limbs. The remaining 20 per (*;Mit were 

 caused by wounds of various kinds. Pickers, in climbing trees '^or.J'r^it, 

 bruise the bark with their shoes. In cultivating, the trees are often in- 

 jured by singletrees and niachiiiery striking (ho limbs. The injury known 

 as "sun scald" often is an open door for infection. 



