60 PEOTECTIOX OF PLANTS, 1921-22 



Causal organism 



The pathogen is a member of the Adelomvcetes (1), order INloniliales, family 

 Dematiaceae, genus Alternaria. The writer has been unable to identify it with 

 any species of this genus previously described and suggests tentatively the name 

 Alternaria polypodii n. sp. with the following characters: — 



Mycelium hyaline to light brown; conidiophores erect, simple or sparsely 

 branched, septate, usually darker than the vegetative hj-phse, 24.5-42.5 x 1.7-4.4 

 u; conidia clavate, usually broade.-^t in the middle, septate mostly transversely 

 but also longitudinally, muriform, with long hyaline beak at the apex,caten- 

 ulate, olive to dark brown in colour, measuring 24.5-70.0 x 10.5-19.25 u, (40.3 

 X 13.6 u). 



Variation in spore sizes. — The nature of the substratum has considerable 

 influence on spore size. To make allowance for this factor the organism was 

 grown on iris leaf juice agar, oatmeal agar, and Richard's solution agar and 

 careful measurements made of the spores produced on these media in addition 

 to those occurring on the host plan. The range and average on each medium 

 follow : 



Lesions on Polypodium 31.5-70. 0x10. 5-19. 25u 



(47. 6x14. Ou) 



Iris leaf juice agar PH 5.9 24. 5-52.5x10. 5-15.75 u 



(36. 7x13. lu) 



Oatmeal agar PH 6.5 28. 0-52. 5x10. 5-17. 5 u 



(37.4x13. 9 u) 



Richard's solution agar.. ..PH 5.2 31.5-52.5x10.5-17.5 u 



(39.5x14.3 u) 



The iris leaf juice agar is an infusion of iris learv^es in distilled water with 

 2% dextrose and 2% agar. The oatmeal agar contains 50 grams oatmeal, 20 

 grams agar, and 1000 cc. of distilled water The last medium was made by 

 adding 2% agar to Richard's solution. 



Pathogenicity 



Isolations were made by sterilizing the outer surfaces of small bits of dis- 

 eased tissue in mercuric chloride (1 :1000), washing in sterile water and plant- 

 ing them in agar. In most cases the Alternaria described above was the only 

 organism which developed. The hyphce could be seen plainly growing out 

 from the edges of the diseased tissue. Since this fact seemed to point to the 

 pathogenicit}' of the organism, a number of inoculation experiments were carried 

 out on the host plant. 



A suspension of spores was spraj^ed, by means of an atomizer, on the fronds 

 of the fern. Onlj' a few minute lesions resulted and no Alternaria could be 

 isolated from them. Belie\4ng that the lack of infection was due to unfavour- 

 able moisture conditions, measures were taken to control this factor. A j^oung 

 healthv frond was washed in mercuric chloride and sterile water and then enclosed 



