372 Minnesota Plant Diseases. 



very small clusters, which arise in great numbers, chiefly on the 

 under surface, but also on the upper surface of the leaf. These 

 sometimes coalesce to form larger spots. The clusters are at 

 first closed by the epidermis of the leaf but later break out and 

 expose a dark-brown powder of summer spores. The winter 

 spores are two-celled and are produced in black clusters. They 

 are not, however, common in this country. On account of this 

 latter feature the disease should not be very difficult to combat. 

 It is said to tend to disappear of itself after a regular run of a 

 given greenhouse. Great care should be exercised in preventing 

 the introduction of the disease into a greenhouse. This can be 

 done by close examination of all purchased cuttings and plants 

 and by carefully watching them for some time after their intro- 

 duction. The diseased plant parts should be promptly removed 

 and burned. In case of a persistent attack, "every leaf and stem 

 above ground should be destroyed at the end of the flowering 

 period and the young plants or cuttings, for the next season's 

 supply, be grown in an uncontaminated house, and, if possible, 

 fromuncontaminated material." (Ind. Ex. Sta. Bull. 85 1900.) It 

 has been reported that inside cultivation in summer and selection 

 of rust-free stock will be sufficient to keep the disease in check. 



Rust of hollyhocks and mallows (Puccinia mdvaccarum 

 Mont.). The mallow rust attacks members of the mallow fam- 

 ily, e. g., hollyhocks, marsh-mallows and the small, wild, creeping 

 mallow. It produces only winter-spores which can germinate 

 without a resting period. The spore groups occur in great 

 abundance on the leaf-stalks and leaf-blades and even on the floral 

 parts, causing deformation of these parts. In some cases the 

 plants are killed by the parasite. 



The mallow rust has an interesting history. It was intro- 

 duced from South America into Europe by the Spanish about 

 thirty-two years ago. Before this time it was unknown in Eu- 

 rope or North America. It spread in Europe with remarkable 

 rapidity, growing on cultivated and wild mallows, and is at pres- 

 ent an exceedingly abundant and dangerous enemy to mallow 

 growers. It is said to have completely exterminated mallows 

 in many regions. This disease has been introduced from Europe 

 into the United States and is fast gaining ground. It has not yet 

 been reported from Minnesota but will probably reach this state 

 in due time, if not already here. 



