696 FUNGI 



fungus, infection of seedlings grown on soil which has previously 

 carried a diseased crop, is mainly due to the oospores which 

 are produced in thousands and remain on, or just below, the 

 surface of the soil after the dead plants have completely decayed. 



Pythium de Baryanum is not only a parasite but it is able 

 to carry on its existence as a saprophyte. 



Prevention and Remedy. — (a) Sow thinly and avoid ex- 

 cessive dampness of the seed-bed. 



Usually much more water is given to seedlings growing 

 indoors than is necessary for their vigorous growth. The fungus 

 is specially invigorated by moist conditions and its hyphse 

 more readily penetrate plants containing superabundance of 

 water. 



(^) Avoid shade for the seed-bed and provide for the circulation 

 of air among the seedlings. 



{f) Carefully take up with a certain amount of the surrounding 

 soil and burn all plants as soon as the disease is observed. In 

 this way the spread of infection to the remaining plants may 

 often be averted, whereas if the fungus has become established 

 in the seed-bed it is almost impossible to curtail its ravages. 



(d) Soil upon which ' damping-off ' has previously been noticed 

 should not be used as a seed-bed ; in cases where this practice 

 cannot be carried out, burning refuse on the surface of the land 

 tends to destroy the fungus and its oospores. 



(«) Deep-ploughing so as to bury the upper layers of the soil 

 containing the oospores is beneficial. 



Ex. 280. — Sow seeds of cress thickly in a box or flower-pot containing 

 garden soil, and when the young plants are up, water ihem often. In this 

 way specimens which are 'damping-off' may generally be obtained. 



Or, procure ' damped-off ' specimens of young seedlings of other plants 

 from a gardener. 



{a) Take up some of them and carefully wash away the adhering soil. 

 Observe with a lens the shrivelled part of the stem where the seedlings 

 bend over. 



