502 THE farmers' handbook. 



In recent years a large amount of laboratory work has been done with 

 Burgundy mixture with a view to determining the proportions that would be 

 absolutely the best in practice, but the compounds obtained are very 

 complex, and finality cannot be said to have been reached. The proportions 

 recommended above have so far given satisfactory results in this State. 



Leaf Spot, or Early Blight. 



" The cause of this disease is a fungus, Altemaria solani (or Macrosporiuvi 

 solani), which is capable of living as a saprophyte (that is, upon dead 

 organic matter) in the soil, or as a parasite, attacking the living potato 

 plant. Apparently as a saprophyte the fungus exists in the soil, perhaps 

 quiescent enough during the winter, but developing with the warmer weather 

 up to the point of producing spores which the wind or other means of trans- 

 port may carry into potato plants. Here the spores germinate, forming the 

 usual filaments or hyphse which penetrate the openings (stomata or breathing 

 pores; in the skin (epidermis) of the leaves, and extend within them between 

 and through the tissue cells. In this way the fungus comes to permeate the 

 leaves and stem, but it does not invade the tubers, which are affected by the 

 disease only in a secondary way owing to destruction of the above-ground 

 foliage. In course of time the fungus produces spores, which are borne on 

 hyphae pushed out from the leaf. They continue to be formed on old dead 

 pieces of potato plants in which the fungus may remain actively living 

 owing to its saprophytic capabilities. Being dropped upon the soil, or 

 reaching it with bits of decayed plant, the fungus passes there its winter 

 period of rest, to rouse itself and form spores against next season as already 

 mentioned. Since the tubers are exempt from attack the fungus is not 

 carried in them; the danger as far as they are concerned is the possibility 

 of spores being transported in the soil or dust which clings to them. 



As one of the names signifies, the disease may appear early in the season 

 upon quite young plants, but is apt to become more common and destructive 

 as the season advances. The plants are said to be liable at the blossoming 

 period, when approaching their natural loss of vigour; quite healthy strong 

 plants possess considerable resistance. When attacked the leaves first show 

 small more or less circular brownish patches which slowly enlarge and join 

 together and cover the greater part of the leaf, the affected tissue becoming 

 brown, withered, and brittle. The surface of these areas is marked with 

 fine wrinkles which form imperfect rings lying one inside the other like 

 markings on a target. The diseased leaves curl up, especially at the tips and 

 margins; the affected parts lose their healthy green colour and become 

 sickly-looking and yellow. The stems may remain free, and stand defoli- 

 ated after the destruction of the leaves; but it is said they may be attacked 

 first, the disease working upwards from near the ground. Whichever be the 

 first point of attack, the end is usually the death of the whole plant. The 

 tubers show no sign of disease, but will only be in such a state of maturity 

 as they may have reached when the foliage perished, since at that period the 

 food material supplied to them by the leaves is cut off, and they cannot 

 further grow. Thus, whilst the disease itself does not attack them, their 

 incomplete development spoils them for market and exposes them to rot 

 from other causes. 



The preventive measures consist of the avoidance of infected ground and 

 of seed from infected or suspicious sources, and spraying with Bordeaux 

 mixture before the disease is prevalent. As far as possible all diseased 

 material should be burned. The chance of cleaning the soil demands that 

 it be kept free from potatoes for several yoarp. 



