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A Root Disease of Persian Walnut

G. FLIPPO GRAVATT, _U. S. Plant Industry Station, Beltsville, Md._

On three recent trips to southern Europe I noted large numbers of dying
Persian (English) walnuts, _Juglans regia_, in France and Switzerland
and scattered trees in other countries. Dying of Persian walnuts from a
root disease of undetermined cause has been reported from various
European countries for many years. The extensive dying of mature Persian
walnut in a number of areas in southern France is very serious. Farmers
and orchardists are discouraged from planting the Persian walnut even
though it is a very profitable tree when not attacked by the root
disease. In area after area I noted that the farmers had scattered their
Persian walnut trees, separating them as much as possible or planting
them along the boundary of fields instead of in orchard plantings. They
had found too frequently that solid plantings of walnut die from the
root disease. The total number of Persian walnuts in southern France has
decreased alarmingly in the last sixty years. In Tessin Province in
Switzerland many unhealthy Persian walnuts were noted this past summer
showing the same symptoms as in southern France.

Studies By French and Italian pathologists have indicated that the
fungus _Phytophthora cinnamomi_ is the most likely cause of this dying
of walnuts. I was informed that it is worse on soils inclined to be wet
or poorly drained at certain times of the year, conditions favorable for
attack of many hosts of this Phytophthora. The work reported by B. S.
Crandall and me in Phytopathology, March 1945, showed there was a rather
direct relation between soil conditions and _Phytophthora cinnamomi_
damage to black and Persian walnut seedlings. Long periods of heavy
rainfall were very favorable for an epidemic outbreak of this fungus on
walnut and other nursery stock. Another species of _Phytophthora_, _P.
cactorum_, has also attacked black walnuts in nurseries in eastern
United States