20 DISEASES OF CROP-PLANTS 



infect the host seedlings as they appear. Seedlings beyond a 

 certain size are no longer susceptible. 



In other species the infection is through the flower. In this 

 type the chlamydospores are distributed at the time of flowering, 

 and arriving in healthy flowers germinate there and infect the 

 developing seed. The mycelium remains dormant through the 

 ripening process and until the seed in due course germinates, 

 when it grows up with the developing seedling as previously 

 described. 



The corn smut (see Chapter XXIV) is different from either 

 of these types, being able to infect its host on any tender part 

 and at any age. 



With smuts of the first type, control is easily obtained, where 

 uninfected ground is available for planting, by some form of 

 seed disinfection. Those of the second type cannot be dealt 

 with in this way, and seed from a clean source has to be sought. 



With both types rotation of crops should be practised to 

 reduce infection from the soil, and the inclusion of diseased 

 plants in litter or manure should be carefully avoided. Nor is 

 it safe to feed smutted grain to animals, since a proportion of 

 the spores may pass uninjured through the alimentary canal 

 and be distributed with the manure. 



The Powdery Mildews {Erysiphacece). 



The Erysiphacese is a well-known family of Ascomycetes 

 whose members are strictly parasitic, growing as white webs 

 on the surface of leaves and tender stems, and deriving nourish- 

 ment from the underlying tissues by haustoria (in a few species 

 by short hyphae) which penetrate the cells. 



The family has a wide range of hosts in temperate countries, 

 and species occurring on cereals, grape, hop, etc. give rise to 

 widespread and destructive epidemics. In the Tropics the 

 family is thinly represented and of small account. 



The conidial fructifications, the only stage which appears to 

 be developed in these islands, consist in the great majority of 

 species of short upright hyphal branches bearing a chain of 

 hyaline oblong or rounded conidia. Forms of this type are 

 referred to the genus Oidium, in the absence of the perithecial 

 fructifications on which more definite classification is based. 

 The latter are subspherical bodies formed in the mycelium, 

 usually in autumn, bearing projecting bristle-like or elaborately 

 branched appendages, and containing one or more roundish asci 

 with 2 to 8 spores in each. 



The Oidiums of the grape and rose are common in the West 

 Indies, and there are doubtless other species to which attention 

 has not yet been paid. The leaf mildew of cotton belongs to the 

 closely related genus Ovulariopsis. 



The standard remedy for this class of disease is dusting with 

 powdered sulphur or spraying with a sulphur compound. Narrow 



