382 Minnesota Plant Diseases. 



has been recommended for plants under glass the weak solution 

 will obviate the injurious effects of stronger solutions in retard- 

 ing flowering. Plants in the open field should be sprayed 

 throughout the summer with one-half strength bordeaux every 

 ten days. 



The downy mildew of seedlings (Phytophthora omnivora 

 DeBy.\ This fungus pest is a parasite on seedling trees of 

 many families of plants. It is especially destructive in nurseries 

 where it may destroy seedlings of conifers and many other plants. 

 It is not dangerous to older plants but confines its attack to young 

 seedlings which have produced only a few leaves. It appears 

 first on the stem, cotyledons, or first leaves, as brown or blackish 

 patches. On these patches develops a very delicate film of spores 

 produced in a manner very similar to those of downy mildew of 

 potato. These spores may germinate directly to an infection 

 tube or they may produce swimming spores, just as do the spores 

 of the potato mildew. The parasite shows little choice of host 

 except in the selection of young plants. It is therefore amateur- 

 ish in its style of parasitism, but it is nevertheless very de- 

 structive. In a few days' time it may destroy whole beds of 

 seedlings. It produces winter spores by a breeding act, as is 

 common among the downy mildews. 



When the pest appears in nurseries, if only on a few plants, 

 these can be destroyed. The most effective methods are, how- 

 ever, those of ventilation and drainage. The fungus thrives best 

 in very moist situations and in moisture-laden atmospheres and 

 in shaded positions. By avoiding as far as possible these condi- 

 tions the fungus may be kept in check. As the winter spores are 

 very resistant, diseased plants should be carefully removed and 

 destroyed and plots which have been infected with the disease 

 should not be used for the same purpose for several years. 



The damping-off of seedlings (Pythium debaryanum Hesse.}. 

 Seedlings of plants of the mustard family are particularly liable 

 to become infected with this disease. Many other paints, how- 

 ever, as clovers, corn and a great variety of others, have also been 

 known to suffer from it. Potato plants and potato tubers in 

 storage may be attacked, if the moisture is excessive. The dis- 

 ease usually appears where seedlings are too densely crowded to- 

 gether, or in shady places and where there is excessive moisture. 



