298 CLASS ASCOMYCETEAE 



leads to the buggestion that we should search here for fertilization of a 

 trichogyne by sperm cells. The genus Mycosphaerella contains over 1000 

 species, many of them parasites of great economic importance. The asco- 

 spores are hyaline or pale green with a nearly median septum. The conidial 

 forms are of several types. M. fragariae (Schw.) Lind., which causes the 

 leaf spot of strawberry {Fragaria), has as its conidial stage Ramularia 

 tulasnei Sacc. The brown conidiophores project through the stomata of 

 the diseased spots and produce terminally short chains of cylindrical or 

 rod-shaped hyaline conidia which are produced in acropetal order. In 

 M. sentina (Fr.) Schroet., on the leaves of the pear, the conidial stage was 

 formerly known as Septoria piricola Desm. Here the very long slender 

 hyaline conidia are produced in pycnidia in the leaf spots. The form 

 known as Cercospora cerasella Sacc. is the conidial stage of M. cerasella 

 Aderh. on cherry leaves. The brown conidiophores emerge from the 

 stomata in the leaf spots and bear terminally or almost so single elongated 

 several-celled conidia, tapering somewhat toward the apical end. When 

 one falls off the conidiophore elongates slightly in a sympodial manner 

 and produces another conidium, and so on, until eventually an old conidi- 

 ophore may show the scars of attachment of a number of conidia. Myco- 

 sphaerella pinodes (B. & Bl.) Stone, on the pea, produces pycnidia con- 

 taining two-celled hyaline conidia {Ascochyta pisi Lib.) while M. tahifica 

 (P. & D.) Johns., of the beet, has as its conidial stage Phoma hetae Fr., in 

 which the pycnidia contain hyaline ellipsoidal one-celled conidia. In all 

 these species and others like them the conidial stage is the destructive 

 stage while the perithecia are produced in the dead overwintering tissues. 

 The different types of conidial production have been used by Klebahn 

 (1918) as a basis for segregating the genus into several subgenera, e.g., 

 Ramularisphaerella, Septorisphaerella, Cercosphaerella, etc. 



Family Pleosporaceae. In the Family Pleosporaceae the genus 

 Physalospora has hyaline or pale brown ellipsoidal ascospores. In P. 

 cydoniae Arnaud, which forms its perithecial stage only on dead twigs, the 

 conidial stage is the destructive Sphaeropsis malorum Pk., which causes 

 the black rot of the fruit and the twig blight and canker of the apple and 

 quince. The conidia are produced in pycnidia and are large, ellipsoidal 

 and dark-colored when mature, sometimes becoming uniseptate when old. 

 Venturia inarqualis (Cke.) Wint., the cause of the scab on apple leaves 

 and fruits, forms its perithecia in the overwintered leaves infected the 

 previous summer. Its two-celled, slightly colored ascospores are expelled 

 sometimes to a height of 15 mm. The conidial stage {Fusicladium dendrit- 

 icum (Wallr.) Fckl.) develops subcuticularly on the leaves and fruit. In this 

 species both Killian (1917) and Frey (1924) have shown that a well- 

 developed antherid unites with the trichogyne which terminates the coiled 

 ascogonium and several male nuclei pass into it and by successive disso- 



