Mushroom growers ventilate their houses as much as possible without excessively drying out the layer of soil on the surface of the compost. This practice is based on an obvious depression of growth or "sulking" of the mushrooms and reduced yields when mushroom houses are closed up for several hours. Experiments indicate that two gases given off by the growing mushrooms themselves, carbon dioxide and an unsaturated hydrocarbon gas, can cause toxic effects in tight houses.

Even when the crop is finished and the houses are empty, growers must take precautions against diseases. They heat the houses to 135 and introduce formaldehyde fumes to combat several diseases that otherwise could be carried over from one crop to another.

NEMATODES are a serious problem wherever mushrooms are grown commercially. They are hard to control and cause heavy losses. They may be actually parasitic on the hyphae or "rootlet system" of the mushroom. With other organisms, they may produce toxic substances. Probably they act as disease carriers.

Although nematodes were present in enormous numbers in mushroom compost, they were not known to be parasitic on mushrooms until 1949. For many years, the reduction in yields in some beds after two or three "breaks" and the total lack of production of mushrooms in others could not be satisfactorily explained. The cause of the trouble was believed to be due to a fungus always present on the affected beds until it was pointed out that the fungus is not parasitic on the mushroom hyphae but acts as a predator on nematodes, which are the prime causes of the disease. Subsequent study showed that an undescribed nematode is parasitic on the mushroom hyphae and is responsible for the decreases in yields or the disappearance of the mushroom fungus from the compost.

The nematode has a stylet, or spear, with which it makes punctures in the hyphae. The punctures are avenues of entrance for bacteria, which otherwise would be unable to enter the hyphae. E. J. Cairns and C. A. Thomas in 1950 reported that the failure of many beds in the Kennett Square area of Pennsylvania was due to the activity of nematodes and bacteria in the compost. They believe that the tremendous numbers of both organisms produce metabolic products that inhibit the production of mushrooms but do not visibly injure the hyphae.

Most of the nematodes involved in the complex are free-living, non-stylet forms. The only known way to control the nematodes is to raise the temperature of the compost to at least 140 F. for several hours during pasteurization. The casing soil also should be heat-treated for several hours to eliminate the nematodes in it and prevent their introduction into the mushroom beds.

Nematodes were reported earlier to aid in the dissemination of bacteria responsible for injury to mushroom caps. A free-living and nonparasitic species is believed to be a carrier and distributor of the bacterium Pseudomonas tolaasii, the causal organism of the "blotch" disease of mushroom caps. The nematode also is suspected of disseminating other bacterial diseases of mushrooms. There are no means of controlling it after it has become established in the beds. More ventilation is recommended to lower the moisture on the mushroom caps so that the nematode will not be able to move about so freely and disseminate the bacteria causing the "blotch" and other diseases. It can be eliminated if the temperature of the compost is held at 140 or more for several hours during the pasteurization process. The casing soil must also be treated similarly to prevent contamination of the compost with the nematode.

EDMUND B. LAMBERT, senior mycologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, received his doctor's degree in plant pathology at the University of Minnesota in 1926. He has been in charge of the Bureau's research on the diseases and cultivation of mushrooms since 1928. He represented the Department at an international conference on the scientific aspects of mushroom culture in England in 1949.

THEODORE T. AYERS, a graduate of the Pennsylvania State College and Harvard University, is associate plant pathologist in the division of vegetable crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering.

Cucumber scab.