DISEASES OF CULTIVATED PLANTS 
the gradual invasion of the areas has been overlooked. In many 
other leaf diseases 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 conspicuous leaf-spot of the strawberry, the beet, 
the pea, etc., will be recalled. In these while the leaves are im- 
paired 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 spotting of the leaf followed by the for- 
mation of a separation layer in the leaf tissues between the para- 
sitized 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 memory of many close observers. Like most leaf-spot troubles 
which are strictly due to parasitic fungi, this tomato disease has 
been worst in seasons of abundant rainfall. The same applies to 
the shot-hole disease of the plum and the allied leaf-spot of cherry. 
The explanation appears to lie in more favorable conditions for 
spore germination and for the growth or spread of the parasitic 
organisms which produce the diseased conditions. Biting or sucking 
insects also open the way for the entrance of parasitic diseases. 
(See later pages.) 
LEAF IMPAIRMENT THROUGH FUNGUS COVERINGS 
In addition to the leaf-spots or dead areas in leaves to which 
reference has just been made, we have most noticeable examples 
of the spread of the mycelium of certain powdery mildews over the 
leaf surfaces. Casual observers note that these spread over the 
leaves and stems of roses, over the leaves of lilac, of oak, of peach, 
of grape, of forcinghouse cucumbers, of bean and pea and upon 
other plants. While the development of these fungi or powdery 
mildews occurs often rather late in the season, they are nevertheless 
damaging to the host plant over which they spread. Above and 
beyond the interference with the leaf action the impairment of the 
photosynthetic or sunlight processes of the leaves of the plants by 
which all real increase in substance is made to the plants, these 
mildews develop sucking or penetrating organs of the threads of 
