Nature of Variation in Helminthosporium Sativum

SUMMARY

1.	The conidia and germ tubes of Helminthosporium sativum are multinucleate, whereas hyphal cells are uninucleate to multinucleate, and young conidiophores are usually uninucleate. All nuclei in the conidia produced on the same conidiophore are perhaps derivatives of a single nucleus and hence genetically alike.
2.	True hyphal fusion between races of H. sativum is common but rare between H. sativum and other dark-spored species. However, pseudo-hyphal fusion between species of Helminthosporium is not uncommon.
3.	Cultural comparisons were made of 207 hyphal-tip isolates obtained from 103 germinating conidia taken from colonies giving rise to variants or from mixed colonies on agar drops. All the isolates from a single conidium were identical with the original culture.
4.	Cultural comparisons were made of 524 conidial isolates obtained from 205 individual conidiophores taken from mixtures of 2 distinct races or species growing on agar drops or from colonies derived from barley tissues infected with 2 or more distinct races or species of Helminthosporium. All the isolates from the same conidiophore, with two exceptions, were identical culturally.
5.	Variants occur frequently in Helminthosporium spp. either in the form of sectors or “ patches.” Although H. sativum race 1 has been cultured for 17 years, it still gives rise, under certain conditions, to numerous variants of different types.
6.	Potato-dextrose agar rendered stale by certain bacteria was found conducive to the development of variants.
7.	The results indicate that genetic variation in H. sativum is caused primarily by nuclear change rather than by heterocaryosis.