LETTUCE, ONION, AND PARSNIP DISEASES 405 



The second fungus, named Fusisporium, illustrated at B, almost 

 invariably accompanies the Peronospora. Under the microscope its 

 growth looks like dense fields of transparent wheat. When the 

 separate ears are magnified to four hundred diameters, they are 

 like the illustration at H ; each ear is divided into four parts. At 

 maturity these parts fall asunder into four oblong spores, as at J ; 

 but when these spores fall on a damp place, they speedily take 

 a globular form, as at K, and then they rest for a few days, weeks, 

 or maybe months, and at length reproduce the Fusisporium. Like 

 the last, this smaller fungus decomposes the host plant by contact. 



These explanations show the immense power of increase possessed 

 by these two fungi. The number of reproductive bodies these pests 

 are capable of producing is simply marvellous, and quite incompre- 

 hensible to the mind. When weather conditions tend to destroy the 

 fungi outside the leaves, stems, and tubers, then the parasites carry 

 on their existence in the resting-spore stage, or by perennial spawn 

 or mycelium. 



As the resting-spores and perennial mycelium hibernate in decay- 

 ing Potato refuse, it is of the highest importance, with a view to the 

 prevention of the Potato disease, that no bad Potatoes or foliage should 

 be allowed to rot on the ground or on manure-heaps through the winter. 

 Diseased Potatoes, stems and leaves, should, as far as practicable, be 

 burnt. If burning is inconvenient, deep burying should be resorted to. 



LETTUCE DISEASE. This mould is sometimes of the most destruc- 

 tive character ; it is a close ally of the Potato mould, and is named 

 Peronospora gangliformis. It covers Lettuce leaves with a fine white 

 bloom, and this bloom decomposes the leaves, and makes them adhere 

 together in one putrescent mass. It should be looked for in its 

 earliest stages, and hand-picked and burnt. 



ONION DISEASE. This is also caused by an ally of the Potato 

 mould ; it is named Peronospora Schleideniana, and it is occasionally 

 very destructive to Onions ; in some districts it is almost unknown ; 

 in others it is common. It occurs at uncertain intervals of time with 

 extraordinary virulence, and then utterly destroys the crops. Autumn 

 sowing is considered a good preventive by many growers, as the 

 disease is frequently fatal to spring seedlings. 



PARSNIP DISEASE. This is also caused by a Peronospora, named 

 P. nivea. When it attacks Parsnips, it causes the roots to become 

 putrescent in the style of bad Potatoes. 



