INVESTIGATIONS ON THE RUST OF ROSES PHRAGMIDIUM MUCRONATUM FR.

SUMMARY 1. Cross‐inoculation experiments have shown that Rosa laxa and certain varieties of R. canina are not attacked by Phragmidium mucronatum from the cultivated rose, and that the rose is resistant to certain forms of the rusts on briars. 2. These differences are correlated with differences in the size of the teleutospores. 3. The teleutospores of P. mucronatum require exposure to winter conditions before they will germinate. 4. The mycelium of the rust may persist for several years in the stem but there is no evidence of systemic infection. The writer is indebted to Mr Courtney Page for supplying material and for much valuable information, and to Dr W. F. Bewley for helpful criticism and advice.