Donald Reddick

Building up virulence in phytophthora infestans

SummaryThe virulence ofPhytophthora infestans has been increased by passage through resistant varieties to a point where a number of hybrids which give an immune reaction in comparison with standard varieties were blighted severely. A similar build-up of virulence occurred under wholly natural conditions. A gap exists which has not been bridged either naturally or artificially so that many hybrids still exist which give no reaction whatever when inoculated with cultures of any virulence.Some speculation is indulged in regarding the case of Champion, the potato-tomato situation in North America, and the occurrence of biological specialization.

Development of blight-immune varieties

SummaryThe foregoing essay is essentially an orientation of published facts presented in a fashion designed to give an overall picture of the blightimmunity situation as it exists at the present time. Those places where speculation or indications are introduced are the places where more intensive work is called for. It is to be hoped that more persons will become interested in the general problem of overcoming permanently this century-old menace to potato production and that this statement will be of help to them in overcoming or avoiding some of the dfficulties involved.

Biological specialization in phytophthora infestans

SummaryPhytophthora infestans as it occurs in most parts of North America either is not biologically specialized or else the varieties of potatoes and tomatoes now grown are not kinds that reveal the presence of more than one form. The form prevalent in North America is the same as the one which occurred at Moscow in 1928 and at Munich in 1930. The demonstration of a biotype in Pomerania in 1932 may be attributed to a new form arising from the production of viable oospores of the fungus or to the fact that a hybrid possessingdemissum “blood” serves to bring out the existence of such a type.

Some ecologic relations of the potato and its chief fungous parasite,phytophthora infestans

Summary1.|The susceptibility of the potato plant to attacks ofPhytophthora infestans is not known to be altered by variability of weather conditions encountered during the growing period.2.|The most important external factors in the rapid spread of potato late blight are temperature and humidity.3.|A relative humidity of 95 per cent, or above must be maintained for about 8 hours to permit the production of a germinable sporangium. Liquid water in the form of rain or dew must be present to permit germination, swarming, and infection. Chilling is necessary to stimulate germination of sporangia. When external factors become effective at exactly the proper moment for optimum development of the parasite a very high humidity must persist for a minimum of 11 or 12 hours to permit infection.4.|In seasons when foliage blight is negligible or absent, tuber rot may occur in isolated hills by direct infection of new tubers from mycelium which has grown out into the soil from an affected seed piece. Such affected tubers perpetuate the parasite and make it possible to understand how it is that blight may be widespread following a year of little or no foliage blight.5.|The spread of late blight from a given centre is so slow and the sensitiveness of the sporangia of the parasite to drying is so great, that the appearance of the disease in widely separated areas at about the same time cannot be explained on the basis of migration of the parasite from one area to another.

Seed transmission of potato virus diseases

SummarySeed transmission of the potato disease acropetal necrosis, caused by a virus occurs in a low percentage of cases. Some evidence is presented which indicates that entry of the virus into the embryo may be effected by means of the pollen. The findings are of significance in potato breeding programs where newly produced plants must be grown in the open and especially in those regions whereMyzus persicae is likely to be prevalent.Inferential evidence that acronecrosis and leaf-roll, also caused by viri, may be transmitted through the true seed is also presented.The value of disease-free foundation stock for breeding purposes is indicated.The proportion of healthy offspring arising from diseased parents is so high that a particular plant of great value may be used as either parent in hybridization even though affected with a communicable disease of the virus type.In cases where diseased plants have been employed in a cross, the production of the first crop of seedlings under conditions permitting complete control of vectors of virus diseases is indicated.