HYPOCREALES 



237 



forms. Weese (1914, et seq.) attempts to create developmental series 

 according to the structure of the perithecial wall. Consequently the 

 different genera are differently defined by different authors; all these 

 attempts, however, have not yet afforded a complete system. 



The most important plant pathogens in the genus are: Nectria 

 cinnabarina, a wound parasite in most of our frondose trees and shrubs, 

 causing canker and dieback of the twigs and forming on the dead twigs 

 striking red conidial fructifications, Tubercularia vulgaris (Fig. 152, A, D); 

 N. cucurbitula, a wound parasite of conifers; and N. galligena, the Euro- 

 pean canker of apple (imperfect form, Cylindrocarpon [Fusarium] mali). 

 Often the harmless saprophytic N. ditissima (imperfect form Cylindro- 

 carpon (Fusarium) candidum), which is only pathogenic for beeches, on 

 account of its association with JV. galligena, is often considered as the 

 cause of the canker. In Calonectria, C. graminicola (C. nivalis, Fusarium 



Fig. 152. — Calonectria erubescens. A. Ascospores ( X 750) . B. Mature perithecium with 

 subiculum (X 140). C. Mature asci (X 400). (After Weese, 1914.) 



nivale), the "snow mould" chiefly on rye, causes a wad-like covering and 

 death of seedlings; furthermore in damp summers it appears at the base of 

 the stems and causes a foot rot of grain (Schaffnit, 1912, 1913). Gibber- 

 ella Saubinetii (Botryosphaeria Saubinelii according to the nomenclature 

 of Weese 1919) causes foot disease and scab of small grains; Pleonectria 

 berolinensis causes the death of Ribes. 



Hypomyces differs from the four previous genera in the extensive 

 development of gemmae. It is chiefly parasitic on agarics and forms a 

 stroma on the underside. The infection is generally visible by the 

 appearance of a fine, arachnoid mycelium whose hyphae cut off acroge- 

 nously fine, hyaline, generally unicellular conidia; in this stage, the fungi 

 in question have been assigned to various imperfect genera, as Verticillium, 

 Botrytis and Sporotrichum. Later there appear on the same hyphae, 

 thick-walled, often sulptured gemmae. These two forms were earlier 

 regarded as independent species and assigned to Sepedonium, Mycogone, 



