224 Diseases of Greenhouse Crops 



caused by the stings of aphides, thrips, and red 

 spiders. The irritant injected by these pests causes 

 the cells to react and finally to collapse, resulting 

 in the specking previously mentioned. 



Control. The carnation is a plant which is nat- 

 urally adapted to a dry atmosphere. Under such 

 conditions in the greenhouse aphides, thrips and 

 red spiders are at their maximum activity. To keep 

 these pests in check fumigation with tobacco ex- 

 tracts or hydrocyanic acid gas is resorted to. The 

 use of the latter, however, cannot be recommended 

 for all carnation varieties. 



Rust 



Caused by Uromyces caryophyllinus (Schrank) 

 Wint. 



Symptoms. The rust is readily recognized by ele- 

 vated blisters or sori filled with brown spores. The 

 sori are first covered by the epidermis of the host, 

 but when they ripen the latter bursts open, liberat- 

 ing the mature spores. This disease is more prev- 

 alent in overheated and overwatered houses. In- 

 fection once established will usually destroy a large 

 per cent of the plants and seriously cripple many 

 others. The disease may be found on all parts of 

 the plant except the roots. Carnation rust seems to 

 be more prevalent in the states lying east of the 

 Alleghenies. Few greenhouses seem to be entirely 

 free from the rust. 



The Organism. The fungus has two spore stages, 



