May 1, 1920 Philippine Downy Mildew of Maize 99 



by using diseased plants for fodder, for cattle appear to dislike the taste 

 and will not eat the infected plants unless they are mixed with a liberal 

 proportion of the healthy. 



One of the most serious features of the attack by the downy mildew 

 is that the infected plants are rendered susceptible to the attacks of a num- 

 ber of secondary parasitic organisms which contribute to the destruction 

 of the weakened plants. In the rainy season there frequently occur de- 

 structive rots of the stem, ear, and shank, with which at least two species 

 of Pythium and bacteria appear to be associated, while a species of Hel- 

 minthosporium, which is only occasionally severe on healthy plants, is 

 usually very destructive to plants weakened by downy mildew. 



SYMPTOMS 



The effect of the disease on the corn plant varies greatly with such con- 

 ditions as the age of the plant when infected, the means by which infection 

 takes place, the varietal nature and individual condition of the host, and 

 the environmental conditions which accompany and follow infection. As 

 a result, no small, clearly defined group of symptoms can be described 

 which will entirely cover the effects of the disease on the host. 



In general, however, the disease may be said to manifest itself by the 

 loss of chlorophyll in more or less sharply defined areas of the leaf, by the 

 production of a whitish down of conidiophores principally on the chlo- 

 rotic area, and by a more or less extensive alteration in the form or the 

 normal growth of the plant. The change in color is the most striking 

 and obvious symptom. Since, however, somewhat similar changes in 

 color and form may result from other causes, the characteristic downiness 

 is the surest indication of , the disease. 



The effects of the disease may appear at any time from the putting out 

 of the third or fourth leaf to the formation and maturing of the tassel and 

 ear, but in any case the tissue of the host is thoroughly invaded by the 

 mycelium before any external signs appear. 



When appearing early in the development of the plant the symptoms 

 are as follows: The second, third, or perhaps the fourth leaf, when nearly 

 developed, shows at the base two or three rather narrow, longitudinal 

 stripes of a pale yellow to whitish color (PI. A) with the exception of which 

 the leaf is quite the normal green. However, the two or three leaves 

 already partly developed above this, and all the leaves which subse- 

 quently appear, are almost completely whitish or pale yellow. Moreover, 

 these leaves never attain the normal shape and size but remain much 

 narrower and become rigid, so that they ascend stiffly instead of bending 

 in the normal flexible manner (Pi. A). The growth of the stem is also 

 checked, so that the plant becomes more or less dwarfed. As the growth 

 of the leaf sheaths is not decreased proportionately, they often deeply 

 overlap to form a cover which may inclose and even project beyond the 

 stunted tassel (Pi. 18, A). The root system also is usually affected so that 



