260 THE FUNGI WHICH CAUSE PLANT DISEASE 



Conidia occur as Macrosporium, Alternaria, Cladosporium, 

 Sporidesmium, Phoma, Helminthosporium. 



There are over two hundred twenty-five species, mostly sap- 

 rophytic. Many conidial forms whose connection to this genus 

 have not yet been definitely proved probably belong to it and are 

 in many instances parasites. 



P. tropeoli Hals, is reported as the cause of disease of the cul- 

 tivated Nasturtium.^*" 



Perithecia pyriform, 140-160 fi; asci oval, one-sided, spores 

 hyaline or very light-olivaceous, 25-35 x 6-8 ii. 



The Altemaria-form was grown from the ascospores by Halsted 

 and from the Alternaria spores, grown in pure cultm-e, perithecia 

 were obtained in about twelve days. 



P. albicans Fcl. occurs on chicory as Phoma albicans; 



P. hyacinthi Sor. on hyacinths with its conidia as Cladosporium 

 fasciculare; P. hesperidearum Cotton, in its conidial form, Spori- 

 desmium pyriforme, causes a black mold on oranges. 



P. herbanun (Pers.) Rab. (comdia= Macrosporium commime) 

 is a common saprophyte which sometimes becomes parasitic. 



P. pisi (Sow.) Fcl.^'' is found on the garden pea; 



Perithecia and spores as in P. herbarum but spores more narrow. 



P. ulmi. Fr. causes an elm leaf spot. P. infectoria Fcl. a com- 

 mon saprophyte, parasitizes tobacco. 



P. oryzae Miy. is on rice; 



P. negundinis Oud. is injurious to nursery stock of Negundo; 



P. putrefaciens (Fcl.) Fr. (conidia= Sporidesmium) is on carrots. 



Pleosporse on grains.*^^' '^* 



Several species of Pleospora with their attendant conidial forms 

 of Helminthosporium and Alternaria are known on various grains 

 and grasses. Cross inoculation experiments have shown here 

 biologic specialization similar to that encountered among the 

 Erysipheae, in that conidia or ascospores from one host usually 

 give negative iresults on host species other than that on which they 

 grew. Thus Diedicke ^*^ says the Pleospora of Bromus cannot 

 be grown on Triticum repens nor on cultivated barley or oats. 

 Helminthosporium was formerly thought to be the conidial stage 

 of all of these grain Pleosporas, but recent work of Diedicke shows 

 that one form which he regards as P. trichostoma (Fr.) Wint. 



