'50 



Commercial Gardening 



leaves also begin to curl, and in severe cases the stems and leaves become 

 blackened in the course of a day or two especially in warm damp weather 

 and when the Potatoes are planted too close together, as they usually are. 

 Examination of the affected portions with a good lens or a microscope will 

 show white and delicate threads. These are simple or branched stalks or 

 conidiophores, which spring, through the leaf pores (stomata), from the 

 mycelium of the fungus already feeding in the tissues of the leaf. At the 

 tips of these delicate branches egg-shaped and colourless sacs, called conidia, 

 are borne. From each of these conidia, when ripe, a number of zoospores 

 are distributed. Each zoospore in due course germinates under the con- 

 ditions mentioned, and sends a germ tube into the leaf tissues either 



. Peronospora (Phytophthora) infestans 



a, Fungus, with spore cases proceeding from stomata. b, Section of Potato legf, showing the mode in which 

 the mycelium creeps amongst the loose tissue of the leaf. 



through a stoma or the epidermis itself. Thus the disease spreads with 

 great rapidity, and not ^nly -are the leaves and stems affected, but the 

 tubers also, if so exposed that the spores fall upon them. 



The usual remedy recommended for the Potato disease is Bordeaux 

 mixture (a recipe for which is given by Mr. George Massee in Vol. Ill, 

 p. 49), and there is no doubt as a preventive it is worthy of recom- 

 mendation. The commercial aspect of spraying with Bordeaux mixture 

 may be gleaned from the following statement of Mr. A. W. Sutton, who 

 carried out an experiment on two plots of Magnum Bonum Potatoes one 

 being sprayed, the other not. One plot was sprayed three times, the other 

 not at all. The effect was very marked. The growth of the sprayed plants 

 continued some time after the unsprayed portion had died down. The 

 weights of the two plots, when lifted, were: Sprayed, 3 cwt. 1 qr. 25 lb., 

 Unsprayed, 3 cwt. 1 qr. 4 lb., a balance of 21 Ib. in favour of the sprayed 

 plot. Strange to say, the quantity of diseased tubers was the same in both 

 plots, viz. 4 lb. "It is therefore a question", says Mr. Sutton, "whether 

 the additional weight per acre would compensate the -grower for the some- 



