RUST OF ANTIRRHINUM. 41 



The uredinia on the stem are much elongated. Here the ruptured epi- 

 dermis is more noticeable than on the leaves. Uredinia on the stem 

 usually occur at the base of a petiole, or at the crotch of two branches, 

 or any place where water may stand. It is the girdling of the stem by 

 uredinia which causes the branch or plant to wilt and die. It is not 

 especially common, however, for snapdragon rust to cause the death of the 

 host plant. 



The telia are black, not brown. They are leathery, not powdery, and 

 must be scraped off if they are to be removed. Telia are more common on 

 stems than on leaves, but are not numerous anywhere. They are slightly 

 smaller than the uredinia and are usually somewhat sunken, with the 

 ruptured cuticle projecting above them. Teliospores are sometimes borne 

 in the same sorus with the urediniospores, but the telia may be distin- 

 guished macroscopically by their blacker color and harder consistency. 



In the greenhouse the disease occurs at all seasons of the j^ear, but is 

 more serious and conspicuous during April and May. 



Causal Organism. 

 Morphology. 



Snapdragon rust is caused by the fungus Puccinia Antirrhini Diet, and 

 Holw. The mycelium of the fungus occurs chiefly between the spongy 

 parench5Tiia cells of the leaf and between the cortex cells of the stem. 

 It is more abundant in the leaf than in the stem. It is colorless, septate 

 frequently, and branches profusely. It is intercellular and provided with 

 haustoria (Fig. 4, Plate 2) . The haustoria are constricted at the point of 

 entrance to the cell. Within they become broader and vase-shaped, or 

 bear short knoblike branches. A dilute solution of eosin makes the 

 haustoria easily visible. A cross section through an infected leaf reveals 

 beneath each uredinium a stroma of interwoven mycelium (Figs. 1 and 

 5, Plate 2). This stroma underlies the whole sorus, and extends in a 

 ring around its edge. From this stroma the spore-bearing hyphse arise. 



Two types of spores are known in the life cycle of the fungus, viz., 

 urediniospores and tehospores. The urediniospores are spherical to 

 elliptical. They are 22 to 30 microns in length and 21 to 25 microns in 

 diameter. They are borne on pedicels of varying length from which they 

 become detached at maturity. The urediniospores are yellowish brown 

 in color. Their walls are provided with short spines and have two or three 

 germ pores. The teliospores are 36 to 50 microns in length and 17 to 26 

 microns in diameter. These spores vary greatly in shape (Fig. 2, Plate 2) . 

 The apex may be sharply pointed, rounded or truncate ; the base is usually 

 attenuated, but may be rounded off bluntly. There is a shght constriction 

 at the septum. The epispores are dark brown to black, and the wall is 

 smooth, possessing no such spines as occur on the urediniospores. Each 

 of the two cells of the teliospore is provided with a germ tube which is 

 apical in the terminal cell and occurs just below the septum of the basal 

 ceU. 



