The treatment of large old lesions is usually of doubtful value, particularly if there is evidence of staining in the wood or if the top of the tree shows signs of deterioration. Also, if trees show many active lesions over the trunks and limbs, it may be that too much attention and labor will be required to warrant treatment.

A new and satisfactory treatment is to induce bark scaling with chemicals. It consists of brushing the lesions and marginal bark with a 1 percent solution of dinitro-o-cyclohexyl-phenol in kerosene. The solution, known as DN-75, is prepared by the Dow Chemical Co. It is inexpensive and easily applied, but the directions as to its use and application must be carefully followed. Improper use of DN-75 will seriously injure the trees. The chemical, like the scraping treatment, is not a cure. It is merely a way of prolonging the commercial productivity of the tree. It is not recommended for trees that show top deterioration or for trees that already show wood staining under the lesions.

PSOROSIS, primarily the bark-scaling type, psorosis A, is widely distributed over the citrus-growing areas of the United States and elsewhere. Its economic importance varies in individual plantings. In California the percentage of diseased trees may be as high as 75 percent in groves not more than 25 years old.

Data obtained in 1943 and 1944 in a survey conducted in California by the Emergency Plant Disease Survey covered 18 groves-14,320 trees-16 to 50 years old. The percentages of trees with bark symptoms ranged from less than 1 percent to approximately 30 percent. Replants, thought to have been necessary because of psorosis, sometimes ran as high as 15 percent in the older groves. The surveyed groves perhaps represented the average of the California plantings. Many plantings, however, have a much higher percentage of diseased trees than the ones surveyed. Yield estimates in the older groves showed that the affected trees were yielding on the average about one-third less than healthy trees of the same age. As the effect of the disease worsens, the yields drop proportionately. Replanting also represents a loss.

OTHER VIRUS DISEASES of citrus include stubborn disease and exocortis. Stubborn disease was first called that at East Highlands, Calif., by J. C. Perry, who observed that when affected navel orange trees were top-worked with healthy buds of the same variety they were slow or stubborn in their growth and had the same general appearance as the original trees.

H. S. Fawcett demonstrated later that the disease could be transmitted by tissue grafting. From his studies he concluded that the disease was caused by a virus. Further observations by J. C. Johnston, L. J. Klotz, and Dr. Fawcett proved that the "acorn-shaped" or "pink nose" fruits sometimes found on navel trees were also symptoms of stubborn disease. The disease of grapefruit in Arizona and the Coachella Valley of California known as crazy top, blue nose, or blue albedo may be the same malady. It is not known that this type of disease occurs in other citrus areas of the world. The little-leaf disease in Palestine is said to resemble the stubborn disease in California.

Its symptoms on foliage and branches are hard to describe. Large affected trees look like the unproductive "Australian" type of navel. The leaves are shorter, broader, and more upright. Multiple buds and shorter internodes tend to make the trees somewhat bushy. At first there are more leaves than normal, but later in severe cases the leaves become somewhat chlorotic and shed more than on healthy trees. Many twigs die and the trees gradually bear less and less fruit.

Fruits on affected trees are usually fewer in number, more irregular in size, and paler in color. Often more off-bloom fruits are produced. Some of the fruits, even the smaller green ones, may show the characteristic acorn shape. Mature acorn fruits, however, are not always found every year on diseased trees. The rind of the acorn fruit appears normal near the stem end and abruptly becomes thinner and smoother until it is quite thin at the stylar end, which, in navels, often takes on a pinkish color hence the name pink nose. Grapefruits frequently have a blue color in the albedo of the thin part of the rind. The blue may be visible on the rind surface hence blue nose.

The disease has not been observed on citrus other than navel orange and grapefruit. There is no definite evidence of natural spread in the field, but lately it has seemed to increase in some older plantings. Because of the suspicion that stubborn disease may be spreading by some natural means from diseased to healthy trees, we recommend that particular attention be given in the selection of budwood parent trees so as to avoid the use of buds from trees that show any suggestion of the disease.

EXOCORTIS, known as scaly butt in Australia, is characterized by a scaling of the bark of trifoliate orange (Poncirus trifoliata), a commonly used rootstock for certain citrus. Usually a severe stunting of the scion or top of affected trees occurs. Navel and Valencia orange, grapefruit, and lemon trees grown on trifoliate rootstock commonly show bark shelling of the trifoliate and a stunted top growth, but sometimes trees are stunted without evidence of bark shelling on the rootstock.

Studies in Australia suggest that the disease is caused by a virus but that it is typically a bud-union effect. It has not been seen on unworked trifoliate seedlings. In two instances the scaling persisted and enlarged onto sucker growth from the trifoliate rootstock that developed after the navel orange scion was broken off or died back completely.

Other experiments indicate that many orange, grapefruit, and lemon trees on other rootstocks carry the virus without showing evidence of its presence. When buds from such trees are propagated to trifoliate rootstock, however, many of the trees grow poorly in the nursery and develop bark Symptoms on the rootstock within 4 to 8 years after budding. On the other hand, it has been established that buds from certain orange and grapefruit trees develop normally and probably remain so indefinitely. No lemon trees in Australia have been found to be free of the virus.

Although exocortis has not been of great economic importance in the United States because of the rather limited use of trifoliate orange as a rootstock, it is of interest because of the possibility that some of the new rootstocks that are being developed or selected to replace those that are susceptible to quick decline may prove to be subject to exocortis. In other words: This disease is an additional complication in the program of finding desirable rootstocks for citrus.

We do not know whether the exocortis virus causes any trouble on citrus grown on the commonly used rootstocks other than trifoliate orange. One way to prevent losses therefore is to use rootstock species that are known not to be susceptible. Because of certain merits of trifoliate orange (such as resistance to quick decline, gummosis, nematodes, low temperatures, and possibly drought, as well as its beneficial influence on yields and fruit quality), however, it has been a preferred rootstock in some areas. Where it is used as a rootstock, the scion bud to be grown on it should be selected from trees on trifoliate rootstock that are 10 years old or older and show no evidence of stunting or bark shelling on the root-stock. We believe that such a practice will result in healthy trees, because we have no definite evidence that trifoliate seedlings carry the virus of exocortis or that there is any natural spread from diseased trees to healthy trees in the orchard.

J. M. WALLACE is a plant pathologist in the Citrus Experiment Station of the University of California at Riverside.

T. J. GRANT, principal pathologist at the Subtropical Fruit Field Station, Orlando, Fla., spent 3 years in Brazil on tristeza disease investigations. Before that he was agricultural attache of the American embassies in Nicaragua, Costa Rica, and Panama. He has been concerned with tropical agriculture and has worked in the Department of Agriculture for more than 17 years. He has degrees from the University of Massachusetts and the University of Wisconsin.