DETAILED ACCOUNT OF SPECIFIC DISEASES OF PLANTS 477 



The unicellular spores measure 10 to 14/i in length. No practical 

 method has been devised for controlling the alfalfa leaf-spot disease. 



Rust {Uromyces striatus Schrot.). — The aecidiaof this rust are found 

 on Euphorbia cyparissias in Europe and in Great Britain the uredinea 

 and telia occur on a clover Trifolium minus. In California, it forms 

 reddish-brown, dusty pustules on the surfaces of alfalfa leaves and 

 in wet weather it may be destructive to the crop, but in dry weather 

 it usually disappears. The spots are on close examination seen to be 

 cinnamon-colored, due to the presence of globose to ellipsoid, faintly 

 echinulate, yellowish-brown uredospores, which measure 15 to 22// 

 with a spore wall i to 2^ thick, and with four to six germ pores each 

 with a small cap. The telia are darker in color, and the teliospores are 

 globose to ovate with a minute papilla striated from apex to base with 

 lines of brown warts and measure 18 to 24 by 15 to 20/x with an epir 

 spore 1 3^ to 2/i thick. The best way of combating this disease is to 

 cut and burn badly affected crops. Frequent close mowing is useful 

 in checking leaf-spot. 



Apple {Pyrus malus L.) 



Bitter-rot {Glomerella cingulata (Stonem.) S. & V. S.). — This 

 fungus, which in some text-books is known as G. rufomaculans (Berk.) 

 Spauld. & von Sch., causes one of the most serious losses in the apple- 

 growing districts of the United States (Fig. 190). It is distributed 

 widely, particularly eastward of the arid portions of the country and 

 its effects are seen during July and August and later, especially during 

 warm rainy weather, which produce sultry conditions of the atmos- 

 phere, when the age of the fruits is such as to render them especially 

 susceptible. Cold weather acts as a check to the spread of the dis- 

 ease. The fruit is attacked chiefly, but the branches may also become 

 diseased. 



The disease first appears as a small brown spot beneath the skin of 

 the apple, which increases gradually in size, keeping nearly a circular 

 outline with a well-defined margin. The central part of the spot soon 

 becomes sunken and this is accompanied by the spread of the fungus 

 throughout the fruit and the formation of pustules. Decay soon sets 

 in and the products of the decay are invariably bitter. The fruits, if 

 attacked on the tree, later fall off, but sometimes, they hang on and 

 become mummified. Two stages in the life history of the fungus have 



