The disease was first recognized in California in 1929 on apple trees growing in Santa Cruz County. In 1938 it was known in 8 counties and in 1947 in 15 Alameda, Butte, Contra Costa, El Dorado, Monterey, Napa, Orange, Riverside, San Benito, San Bernardino, San Joaquin, San Mateo, Santa Clara, Santa Cruz, and Sonoma. In 11 of them in 1953 the disease was restricted in distribution.

Probably it had been introduced on nursery stocks. It has been seen in two nurseries in central California. Because the disease is most prevalent in apple and pear orchards in California and most apple and pear seedling stocks before 1920 were imported from Europe, the fungus may have been introduced into California on such stocks.

The above-ground symptoms consist of sparse foliage, slow growth or none at all, wilting of leaves, and death of twigs, branches, and leaves. The dead leaves may remain attached for several months. Sometimes, particularly when moisture and temperature are favorable, the fungus appears for an inch or so above ground on the bark of the dead tree and forms a dark, velvety mat with white-headed spore-bearing structures, called coremia.

The symptoms below ground are more distinctive. In early stages the white, cottony mycelium pervades the cortex, wood, and adjacent soil. Later the fungus may form indefinite plaques within the bark and loosely aggregated strands of associated hyphae. It never attains the well-defined mycelial fans and less commonly seen rhizomorphs produced by the oak root fungus, Armillaria mellea. Still later the root is killed, and the surface is often covered by a dark, hard, mycelial mat, from which the coremia and cottony white growth may arise.

Dematophora root rot differs from that caused by Armillaria in its erratic distribution within the orchard, in the cottony outgrowth from the root under highly humid conditions, in the absence of a mushroom stage, and in absence of well-defined mycelial fans, or rhizomorphs.

A useful diagnostic feature, when a microscope is available, is a pear-shaped swelling near the end of each cell in the hyphal strands of the fungus. We have not found these in any other fungus associated with apple Foots. The mycelium has a remarkable resistance to competing soil micro-organisms and to dryness. It has survived in apple roots in the laboratory for at least 8 years with only occasional watering.

Once established in an orchard, the fungus seems to spread primarily by two means along and within the roots of infected plants and in fragments of roots, which are sometimes transported by cultural operations. The mycelial growth in soil is usually limited to a few inches from roots or other invaded plant material. Perhaps water spreads the fungus also. No spore form seems to play any part in dissemination of the fungus in California.

To find out which species of deciduous fruit and nut trees are susceptible to attack by the Dematophora fungus, tests were made in two naturally infested orchard plots and in large boxes in which plants were grown in artificially infested soil. Much of the work was done by Harold E. Thomas at the University of California College of Agriculture.

The following plants are susceptible: Quince (Cydonia oblongs), Black Mission fig (Ficus carica), butternut (Juglans cinerea), northern California black walnut (3. hindsii), eastern black walnut (J. nigra), apricot (Prunus armeniaca), mazzard cherry (P. avium), sand cherry (P. besseyi), sour cherry (P. cerasus), almond (P. amygdalis), peach (P. persica), black cherry (P. serotina), Siberian crab apple (Malus baccata), apple seedlings of Golden Delicious and McIntosh (Malus pumila), apple rootstocks known as East Melling 1, 11, IV, V, VII, and IX, and the pear species Pyrus betulaefolia, P. bretschneideri, P. calleryana, P. communis, P. communis cordata, P. regeli, P. ovoidea, P. amygdaliformis var. persica, P. phaeocarpa, P. serotina, P. serrulata, P. ussuriensis.

Other possible rootstocks, especially for apple and pear, are being tested for resistance. The crab apples, Malus floribunda and M. toringoides, in preliminary tests seem to have considerable resistance but little is known of their suitability as rootstocks for the locally grown apple varieties.

Resistance tests conducted for more than 10 years have revealed two important resistant stocks in the species Prunus cerasifera and its hybrids, the myrobalan and the Marianna plums. Selection number 29 of the myrobalan plum and 2624 of the Marianna have proved highly resistant and are good stocks for commercial varieties of plums and apricots. These stocks, especially Marianna 2624, also are resistant to armillaria root rot and to the effects of water-logging of heavy soils. Replanting affected spots with Marianna 2624 rootstock on which plum or apricot can be grown offers the best promise in situations to which these fruits are adapted.

MANY CHEMICALS have been tested against this fungus in closed cans of soil and several have been tried in orchards. To date no encouragement has come out of these tests.

Every possible precaution should be taken to prevent the spread of the fungus during irrigation or other cultural operations as well as in the movement of nursery stock.

H. EARL THOMAS is professor of plant pathology in the University of California, and plant pathologist in the California Agricultural Experiment Station at Berkeley. He is the author, with H. N. Hansen and Harold E. Thomas of The Connection Between Dematophora necatrix and Rosellinia necatrix, Hilgardia, volume 10, pages 561-564, 1937, and Dematophora Root Rot, Phytopathology, volume 24, Page 1145, 1934.

STEPHEN WILHELM is assistant professor of Plant pathology in the University of California and an assistant plant pathologist in the California Agricultural Experiment Station.

NEIL ALLAN MACLEAN was formerly assistant Professor of plant pathology at the University of California and assistant plant Pathologist in the California Agricultural Experiment Station at Berkeley.