ON FORMS OF THE HOP (HUMULUS LUPULUS L. AND H. AMERICANUS NUTT.) RESISTANT TO MILDEW (APHAEROTHECA HUMULI (DC.) BURR.). IV

Abstract

SUMMARY. Several forms of H. Lupulus with yellowish‐green (“golden”) leaves exist. One form (T) has proved persistently immune to mildew both in the greenhouse and in the open. A second form (9) has proved slightly susceptible when grown in the greenhouse. A third form (3) is susceptible in the greenhouse and in the open. This ♂ form appears to be the one found originally in Germany and described under the name of H. Lupulus aureus. No account of the origin of any ♀“Golden hop” hag been found in horticultural literature. Certain seedlings raised from the immune ♀“Golden hop” (the ♂ parent being unknown) possess green leaves and are immune to mildew when grown in the greenhouse. Different seedlings of the wild hop (H. Lupulus) have distinctive physiological or “constitutional” characters, which are constant under the same environment. These characters confer immunity or susceptibility, or intermediate grades of susceptibility, on the respective seedlings. The immunity is retained by the plant after four years' residence in the manured soil of the hop‐garden. Certain seedlings of the wild hop which show persistent immunity when grown in the greenhouse show some degree of susceptibility when grown in the open. The susceptibility shown is usually very slight. There is some evidence that this breaking down of immunity is due to the effect of certain climatic conditions. In the great majority of cases, the greenhouse conditions do not have the effect of making seedlings of the wild hop immune to mildew. In the case of some seedlings extreme susceptibility is shown under greenhouse conditions. The phenomenon of semi‐immunity is shown by certain seedlings of various origins. A form (8) of H. americanus Nutt., obtained from the United States, has proved immune to mildew under greenhouse conditions. Under the same conditions several American cultivated varieties proved susceptible.