66 DISEASES OF TREES 



rainy weather the disease spreads rapidly, but dry weather at 

 once retards further progress. The fungus passes the winter 

 in the form of oospores, which are produced in the diseased 

 leaves. During summer the distribution is effected by sporangia 

 and zoospores, as in the case of PhytopJithora. Infection takes 

 place chiefly on the young shoots and leaves, the epidermis of 

 which is but slightly cuticularized. The disease proves the more 

 destructive to vines and grapes the earlier in the season it 

 appears, and especially so when favoured by wet weather. 



It is not improbable that other species belonging to the 

 genera Peronospora and Pythium do injury to young trees. It 

 is especially desirable that investigations be instituted to prove 

 whether Pythium de Baryanum which in crowded beds causes 

 the death of many agricultural plants is also injurious in the 

 seed-beds of dicotyledonous and coniferous trees.* The ' genus 

 Cystopus also belongs to the Peronosporce, the best-known species 

 of which is Cystopus candidus, which produces the white-rust of 

 crucifers. 



USTILAGINE^E f 



Although this order contains only fungi which are parasitic 

 on herbaceous plants, especially grasses, still the diseases which 

 they produce are of sufficient importance to require a short 

 description here. 



In the every-day language of the farmer, Smut is a term 

 applied to the most varied phenomena of disease in plants. 

 In the narrower sense of the word, however, the term is restricted 

 to those diseases which produce a dark brown mass of spores 

 in certain parts of plants, especially flowers and fruits, less fre- 

 quently on leaves, stems, and even roots. In the particular part 

 of the plant that is occupied by the copious mycelia of the 

 smut-fungus this spore-powder is formed by the abscission or 

 abjunction of abundantly developed fungus-filaments, the tissues 

 of the plant itself being almost completely destroyed. 



The mass of spores is either formed on the surface of the 



* [An allied form was exceedingly destructive in seed-beds of Cinchona in 

 Ceylon in 1880-1881. ED.] 



f [For the British forms of Ustilagineae see Massee, op. tit., and 

 Plowright, British Uredinece and Ustilaginecz (Kegan Paul & Co., 1889). ED.] 



