DOWNY MILDEW OF CUCUMBERS 11 



Table 3. — Relation of Infection of Cucumbers to Length of Time 

 Inoculated Plants Remain Wet. 



Number of hours inoculated Percentage of points inoculated 



plants remained wet at which infection occurred 



24 76 



14 71 



12 77 



10 82 



7 47 



6 22 



5 5 



2J^ 



There was no infection of plants which remained wet only two and one-half 

 hours after inoculation. Five hours on a wet leaf was enough to permit some 

 infection, certainly enough to establish the fungus in a greenhouse or field. 

 There was, however, much more infection of plants which remained wet six hours, 

 and the maximum infection occurred on plants which remained wet ten hours 

 after inoculation. 



Cucumber plants which had been inoculated with a water-suspension of the 

 conidia of P. cubensis were placed in moisture-saturated air at the time of inoc- 

 ulation. After 50 minutes, half of the plants were removed to dry air so that the 

 fohage was entirely dry within 20 minutes. Thirty minutes later, they were 

 returned to moisture-saturated air, with plants similarly inoculated, but not 

 dried, and there all remained for 24 hours. No infection resulted in the case of 

 plants which had been exposed to dry air for 30 to 50 minutes. This supports 

 observations made in connection with the germination of conidia, to the effect 

 that conidia which have been in water for a time may be very quickly killed by 

 dry air, much more quickly than are conidia which have not begun to germinate. 



In another series, cucumber plants were inoculated with conidia obtained early 

 in the morning and at intervals of one hour from plants wet with dew in the 

 field. Infection resulted up to the time that the sun dried the dew from the 

 leaves from which conidia were obtained, but not later. It appears from this 

 that infection in nature, in sunny weather and the absence of rain, takes place 

 the same night or the same morning as that in which sporulation has brought 

 the infecting conidia into existence. The same dew or rain (or water of guttation) 

 which induces sporulation may also provide the necessary moisture for germina- 

 tion and infection. This is one reason why downy mildew often spreads so rapidly 

 in the field. 



Rain is unnecessary for infection by P. cubensis. Dew or water of guttation 

 often provides all the moisture on the leaf which is necessary for infection. 

 As has been noted by Clayton (9) and others, downy mildew of cucumber may 

 be severe even in dry seasons. 



When soil and air are sufficiently moist, as they sometimes are in greenhouses 

 in the fall before artificial heating is used, water of guttation escapes from and 

 collects on the margins of the leaves of (-ucumbers. An examination of cucumber 

 leaves naturally infected in a greenhouse showed that more than half of the 

 first infection occurred on the margins of the leaves. In order to investigate the 

 part which water of guttation may play in infection, cucumber plants were 

 inoculated by placing dry conidia on the dry leaves. Plants thus inoculated 

 were grown in soil maintained at 70 to 75 per cent of its water-holding capacity 



