Murphy — Bionomics of the Conidia of Phijtophthora infestans. 445 



that the conidia became coated with the medium, a feature which is also 

 found in examining contaminated clay. 



Effect of moisture on the vitality of conidia in soil. — In order to determine 

 the effect of excessive moisture and lack of moisture on conidia in the same 

 kind of soil, three ordinary flower pots were filled with moderately dry 

 potting loam. In the surface of the soil in each pot there was sunk a small 

 paper cylinder, the diameter and depth of which were each about 2'5 cm., 

 and the cylinders were then filled with loam as uniformly contaminated as 

 possible with conidia from potato leaves. Four days afterwards, when the 

 soil had settled, one pot was watered copiously with a fine rose until water 

 ran freely from the bottom ; another pot was soaked by standing it in water 

 until the soil was absolutely saturated ; the third was left unwatered. 

 Thereafter, until the end of the experiment, no more water was added. The 

 three pots were kept in a cool room, the central portions, including the 

 paper cylinders, being covered with Petri dish lids. Throughout the greater 

 part of the forty days of the experiment the pots maintained different 

 degrees of moisture, the soaked pot being water-logged, and by far the 

 wettest, the watered pot intermediate, and the unwatered pot driest. The 

 contaminated soil was tested periodically by transferring portions of it to 

 slices of potato tubers. 



The result was, as may be seen from Table II, that the dry and moist 

 soils reproduced the disease for forty days, while that in the water-logged 

 pot did so (though rather irregularly towards the end) for twenty-six days . 

 Beyond this period, in both cases, the result was uncertain, for while some 

 rot followed after the later inoculations, the blight fungus was not recovered. 

 This feature was noted rather regularly towards the end of several 

 experiments. It may be that the small amount of P. infestans which 

 remained viable set up a weak rot and was soon swamped by saprophytes ; 

 or perhaps, as has been noted several times, old and apparently dead 

 mycelium and conidia have a toxic action on potato tissue, setting up 

 small local lesions which sometimes provide a foothold for other micro- 

 organisms. Thirty-six control inoculations, using similarly treated but 

 non-contaminated soil, gave negative results. It is, therefore, clear that, 

 contrary to the views accepted since the time of de Bary, P. infestans may 

 remain capable of attacking potatoes in the soil, even when the latter is 

 water-logged, for comparatively long periods. The most favourable con- 

 ditions for survival are found under the more or less dry conditions 

 reproduced in the " dry " and " moist " soils. At the close of the experiment 

 there was little difference in the amount of moisture present in these two 

 pots. 



