238 Transactions British Mycological Society. 



spores adhering to the surface become detached into the soil 

 and are there ready to infect any susceptible variety that may 

 be planted in following years. During the war it was impossible 

 to ensure that no seed potatoes were taken from infected land, 

 and hence the disease is now present to a certain extent at any 

 rate in practically every county in England, whereas only a few 

 counties were affected in the year before the war. There is no 

 doubt that the fungus was introduced into the United States 

 some years ago by the importation of seed potatoes. In another 

 class of disease, such as a surface mildew, of which American 

 gooseberry mildew is a convenient example, two types of spores 

 are produced ; one of these propagates the fungus rapidly during 

 active growth in the summer; the other is enclosed in an im- 

 pervious covering and serves to tide the fungus over adverse 

 winter conditions, these spores germinating in the spring upon 

 release from their protective coats. Both types of spores may 

 be distributed by mechanical means in the following manner. 

 The winter "spores" — as one type may be called — adhere to the 

 twigs of the gooseberry bushes and may be inadvertently carried 

 in this way from nurseries to commercial and private gardens. 

 It is customary before gooseberry bushes are despatched from 

 a nursery where mildew has been present, to cut back the ex- 

 tremities of the shoots, but this precaution does not entirely 

 ensure against the resting "spores" being distributed with the 

 nursery stock, because these may fall on to the bud scales 

 or into cracks in the bark. The summer spores, which are scat- 

 tered to some extent by wind currents, may be distributed over 

 wider areas by other agencies. The baskets in which the goose- 

 berries are sent to market have been one such means: it is im- 

 possible to exclude all diseased fruit in packing, the spores from 

 which may be carried with the basket to a new destination, for 

 the empty baskets are not always returned to the place from 

 which they were sent. Spores which adhere to these baskets 

 get blown on to healthy bushes and infect them. During the 

 period when American gooseberry mildew was spreading rapidly 

 in this country it was quite common to find that the first bushes 

 in a plantation to become diseased were those in the neighbour- 

 hood of the packing sheds, i.e. in the immediate vicinity of the 

 place where the baskets returned from the market were stored. 

 Spores of such diseases may also be carried directly by human 

 agency. Persons walking amongst diseased plants may carry 

 away with them spores adhering to their clothes. Upon going 

 to another garden, these spores may fall from their clothes and 

 infect healthy plants. 



The economic losses caused by fungal diseases in general and 

 by those of an epidemic nature in particular are collectively 



