Art. XXVI.—Notes on the Spread of Phytophthora infestans, with special Reference to Hybernating Mycelium

By A. H. Cockayne.

in the Auckland Province, but it has not been heard of since till last year.” For my part, I condider that the epidemic outbreak in Auckland in 1904 was in no way connected with the sporadic ones which occurred previously, The cause can be attributed to the fresh importation of Phytophthora mycelium in imported potatoes. There are only two ways that Phytophthora can possibly have been introduced here—either by resting mycelium in diseased tubers, or by asesual spores or oospores; but as these latter have never been definitely discovered, its introduction by means of them is most unlikely. The asexual spores of P. infestans are naturally short-lived, and are in no way provided with any adaptations to withstand the desiccation that they would undergo in passing though the tropics. Therefore it is almost certain that the introduction of thisdisease into New Zealand was by means of dormant mycelium hidden away in the tissues of affected tubers. This view is greatly strengthened by the discovery, on numerous occasions during the past two years, of varieties of imported potatoes in which the presence of Phytophthora mycelium was clearly demonstrated.

All potatoes which are now imported into New Zealand are carefully examined by the Agricultural Department, and those lines found affected with Phytophthora are at once destroyed.

Action of Phytophthora on Potato-Tubers.

There are still many gaps in our knowledge of the full life history of the Irish potato-disease, and the exact pathological processes that obtain in so-called diseased tubers are but imperfectly understood. That the disease is transmitted from season to season by means of mycelium permeating the tissues of the tubers themselves hass been now abundantly proved. Massee has given the name “hybernating mycleium” to that portion of the vegetative body of a fungus which has the power of remaining quiescent during the dormant period of the host's existence, and which can return to normal development as soon as the host commences to devleop. It follows naturally that those fungi which are able to develop hybernating mycelium do no require the same amount of varied spore-formation as those which are not so equipped. The formation of winter spores be decidedly superfluous, and such fungi can be more specialisedin the direction of producing summer and generally short-lived spores, whose object is to rapidly infect large mases of their hosts, provided the environment is suitable.

The finest example of hybernating myucelium is found, as Freeman has shown, in the fungus affecting the various species of Lolium, especially L.temulentum. In over 70 per cent. of

whence it has originated, I am inclined to combat; although I must admit that in the majority of the cases I have examined secondary infection by bacteria and other fungi, notably Fusarium oxysporum, plays an extremely important part in the rotting that occurs in tubers which have been primarily attacked by P. infestans. It is a well-known fact that tubers bearing the characteristic marks associated with Phytophthora attack often during the whole winter without any trace of rot setting in. When these are examined under a miagnification of about 100 diameters, large amounts of dormant mycleium will be seen in those portions of the tissues of the tubers that abut on the darkened and discoloured areas, which are said to be caused by Phytophtora; and in such cases no other fungi or bacteria will be found associated with the Phytophthora mycelium. This observations gains considerably in significance when it is stated that in those tubers on which an active rot is present resting mycelium can hardly ever be discovered, although there will be an abundance of bacteria and other fungi, both parasitic and saprophytic.

If slices of potatoes showing resting mycelium of Phytophthora infestans are placed in petri dishes, and kept moist in a temperature of about 60⁰ to 70⁰ Fahr., the mycelium will rapidly become active, and in a few days an abundant crop of spores will be developed. This shows that the mycelium is not in a very dormant condition, but is really on the border-line between active and hybernating mycelium. This is a very important point, inasmuch as it shows the liability, under certain conditions of the mycelium to become active even when no growth on the part of the host takes place, and in this particular distinguishes it sharply from the more specialiesed resting stages in the life-history of other fungi, such as many of the Ustilagineœ.

When tubers with the resting mycelium of P. infestans are planted, the fungus develops rapidly through the tissues of the developing plant, and if the weather is humid and warm the mycelium becomes markedly negatively geotropic, and if the conditions remain favourable for its development it quickly makes its way into the leaves, on the under surfaces of which it soon produces an abundant supply of spores, which, blown by the wind, can soon spread infection far and wide. If, however, the weather-conditions remain unfavourable for its development, no spores at all may be produced, and to all outward appearance the potato-plants remain quite healthy. In such a case it is not known wheather the fungus can again form resting mycelium without the intervention of a spore-producing stage, as is done in the case of Lolium temulentum, but I am inclined to consider that such can and often does occur.