DOWNY MILDEW OF CUCUMBERS 7 



Wintering-over of the Fungus in the South and Dissemination North- 

 ward by Winds 



Peronoplasmopara ciibensis is known to live through the winter in Florida. 

 Hume (20) stated that it may be found there on living hosts throughout the 

 year, and Weber^ has recently informed the writer that cucurbits are grown and 

 P. cubensis has been found on them in every month of the year in that State. 



It has been suggested or accepted by several investigators (10, 32, 43, 14) 

 tliat P. cubensis, living through the winter in the South, may be disseminated 

 northward in the spring and summer with the northward advance of the season 

 and the cultivation of the host plants. 



Lacking oospores or other equally long-lived spores, the dissemination of 

 this fungus, either locally or for greater distances, can at present be attributed 

 only to ('onidia. The conidia of the downy mildew fungi are usually thought 

 of as being very short-lived, but Angell and Hill (3) have found that the conidia 

 of some of the downy mildew fungi {Bremia lactucae Regel, Peronospora parasitica 

 (Pers.) de Bary, and Peronospora hyoscyami de Bary) may retain their viability 

 for several days or even weeks. Similarly, Katterfield (23) found that conidia 

 of Peronospora schleideni Ung. may remain vial:)le in moist air for ten days. 



The writer has succeeded in keeping the conidia of P. cubensis alive in moist 

 air for 50 hours, but not longer. Spores which can live that length of time may, 

 however, be carried long distances by the wind. 



Ascending air currents may carry fungous spores, like dust, high into the air. 

 Spores have been found floating in the air at altitudes up to 11,000 feet (41). 

 Such spores, even at much lower altitudes may be carried horizontally many 

 miles by wind before they settle to earth. Thus Pennington (34) found evidence 

 that wind-borne aeciospores (of Cronartium ribicola Fisch.) were carried to a 

 point 110 miles from their starting place. According to Lambert (26), there is 

 enough movement of southerly winds to carry urediniospores of Puccinia gram- 

 inis Pers. from Texas to the spring wheat area in the North in less than three 

 days, and the observations of Wallace (47) lead him to the conclusion that these 

 spores, borne north by the wind from Texas or even Mexico, may be inoculum 

 for wheat in the Northern States. 



The conidia of the downy mildew fungi are no less well adapted to distribu- 

 tion by wind both liecause of their smallness and the great numbers in which 

 they are produced. Weston (49) observed that the spread of the Philippine 

 Sclerosporas of maize followed closely the direction and extent of the nightly 

 air currents. He did not attemjit to determine the precise amount or extent to 

 which the conidia of these fungi were disseminated by winds, but he found that 

 they were carried for at least several miles and concluded that wind is the most 

 important factor in their dissemination. Leach (27), similarly, concluded that 

 infection of beets by Peronospora schactii Fckl. can often be accoimted for only 

 by wind-borne conidia. 



As Lambert (loc. cit.j has pointed out, dust storm analogies and other evi- 

 dence indicate that the wind is capable of carrying fungous spores many miles, 

 or as far as from Florida to Massachusetts. But it is not necessary to postulate 

 the flight of the conidia of P. cubensis the whole length of the Atlantic seaboard. 

 Downy mildew of cucurbits is present most years, as the season advances, in 

 most or all of the Coastal States, and it is entirely possible for the conidia to be 

 borne from Florida to Cleorgia to the Carolinas and so on northward by several 



'Letter of October 10, 1931 from G. F. V^eber of Fla. Agr. E.xpt. Sta. 



