428 IXSECTS AFFECTING VEGETATION 



A similar state of parasitic existence is found in the broomrape 

 tribe whose very small seeds are scattered through the soil. Such a 

 broomrape is well known on hemp, and the same hemp broomrape 

 also attacks tobacco in Kentucky and possibly in our state. We 

 have found another broomrape attacking tobacco in one district of 

 Brown county, Ohio, and the illustration shows its appearance on the 

 tobacco roots. 



When the leaves of a plant are attacked these show the direct 

 effects; the symptoms of parasitic leaf diseases are usually localized 

 injury resulting in spotting and often in browning of the leaf parts. 

 Leaves may dry up somewhat slowly and drop to the earth, and yet 

 the leaf tissues are simply dried up. Such conditions may result 

 from late frost as upon shade, fruit, or ornamental trees. A most 

 interesting case was once studied upon catalpa as a result of a frost 

 in May. In that case the drying up was none the less to be expected 

 at that time. 



An even more interesting case of leaf drying and dropping was 

 upon young catalpa trees in a nursery caused by the attacks of a 

 root-rot fungus, Thielavia. Owing to the death of many of the 

 rootlets and finer roots as a result of the root-rot trouble, the loaves 

 of these young trees dried up prematurely in August and September 

 and the leaves all dropped off. Thus we may have leaf dropping 

 as a result of frost, injury by hail, root impairment or localized 

 parasitic attack. 



LEAF SPOT AXD SHOT-HOLE EFFECTS. 



Leaf-spot symptoms are everywhere abundant and are really of 

 very diverse origin. In any example in which the leaf tissues are 

 locally invaded by a parasitic fungus we may expect evident effects. 

 In the downy mildew troubles there may be wet-rot symptoms when 

 the weather is moist, as in the case of Phytophthora or late blight 

 attacking potato or tomato leaves; after the leaves have become badly 

 diseased they may appear to die very suddenly because the gradual 

 invasion of the areas has been overlooked. In many other leaf dis- 

 eases no such rapid multiplication or reproduction of the parasite is 

 possible and limited dead patches or spots are the result. The leaf- 

 spot disease of alfalfa, the various leaf-spots of apple and the con- 

 spicuous leaf -spot of the strawberry, the beet, the pea, etc., will be 

 recalled. In these while the leaves are impaired as to usefulness they 

 do not perish immediately and one may readily fail to estimate the 

 injury at its real seriousness. In a few leaf troubles we have the spot- 

 ting of the leaf followed by the formation of a separation layer in 

 the leaf tissues between the parasitized and the healthy tissues. This 

 results in shot holes in the leaves, as is so very conspicuous in the 

 shot-hole leaf disease of the plum and less conspicuously so on certain 

 sour cherry trees. These leaf troubles are commonly very evident 

 during rainy seasons and are preventable by spraying the foliage of 

 the diseased plants at repeated intervals, thus keeping a supply of the 

 fungicide on the leaves to arrest renewed spore development. 



An interesting leaf-spot disease of the tomato is sometimes very 

 damaging. This disease seems to have appeared in Ohio during the 



