Many factors other than resistance to foot rot enter into the choice of a rootstock. For example, resistance to cold and adaptability to soil type and fruit quality are important. Rough lemon stock is widely used in Florida because of its adaptability to deep, sandy soils but it is only moderately resistant to foot rot and is not very tolerant of cold. The quality of fruit produced over it is not high. Cleopatra mandarin is rather resistant to foot rot and trees on it produce high-quality fruit, but with some scion varieties production is low.

The matter of rootstock selection has been further complicated by tristeza or the threat of tristeza. That disease has been so devastating to trees on sour orange root that sour is now considered an undesirable stock.

It is impossible to state what rootstocks will be most satisfactory from all standpoints, but it is worthwhile to indicate the direction in which progress seems most likely. Sweet orange will probably be used more than in the past, with emphasis on the varieties that are most resistant to foot rot such as Indian River or Jaffa. Mandarin varieties, such as the fairly resistant Cleopatra, doubtless will receive more consideration. Some selections of Poncirus trifoliata stock are highly resistant to foot rot, and will probably be used. In addition, P. trifoliata is tolerant to tristeza, cold-resistant, and produces high-quality fruit especially on heavy soils.

The second approach to the problem of foot rot control is to provide an environment unfavorable to the growth and survival of the fungus in the soil. That is particularly important when trees must be grown on only moderately resistant stocks.

As the foot rot fungus requires abundant moisture, citrus obviously should not be planted on poorly drained, heavy soils. Drainage should be provided where necessary and irrigation water used sparingly. In areas of heavy rainfall, clean cultivation and trimming up the skirts of the tree promote air circulation and more rapid drying of the top layer of soil.

The root crown is the most critical infection site from the standpoint of loss of the tree. Removing the soil from around the crown roots exposes them to the drying effects of air and sunlight and has been found helpful in combating foot rot. Excavation is most easily accomplished by directing a jet of water under high pressure against the soil around the crown roots until a basin approximately 6 inches deep and about 4 feet across is washed out. The young tree, on rootstocks moderately susceptible to foot rot, should be set high enough that after the soil settles the top lateral roots are barely covered.

Careful inspection should be made of the crown twice a year during the first 3 years of growth and at least once a year thereafter. Covering the crown and a foot of the lower trunk once a year with a water suspension of a one-package (ready-mixed) bordeaux mixture (a spray-dried product, made to about the consistency of house paint) is a good way to prevent infection. Under experimental conditions in Florida, however, bordeaux mixture and other fungicides have not proved effective unless combined with the method of exposing the root crown plus clean cultivation under the trees.

Manipulation of the soil pH for the purpose of foot rot control is probably not advisable in mineral soils or in areas where the soil reaction is naturally close to pH 7.0. To attempt to lower the soil reaction from 6.0 or 7.0 to 4.5 might cause serious nutritional disturbances. But in areas of high rainfall and in organic soils, the soil reaction is frequently as low as pH 4.0 to 6.0. Under such conditions, the soil reaction can often be maintained at pH 4.5 by the use of suitable amendments and fertilizer. Even on organic soils, however, nutritional problems may develop when the soil is maintained at pH 4.5 Or lower. Hence, attempts to control foot rot through altering the soil PH may prove useful in some areas, but should be pursued with caution.

The first step in treatment is to expose the infected bark and wood by excavating the soil if that has not been done. Next, all the diseased bark is removed with a heavy knife and scraper, cutting one-quarter to one-half inch into healthy bark to be sure that all infected tissue is removed. The exposed wood is then covered with a fungicide. Many different fungicidal preparations, such as bordeaux paste, have been used as wound paints and with fairly satisfactory results. However, Avenarius (Red Arrow) carbolineum seems particularly suitable in this instance because it penetrates well into the wood in addition to being an excellent disinfectant and is waterproof for a period of several months. Later the wounds should be painted with a water-emulsified asphalt preparation (obtainable under such names as Tree Seal or De Ka Go), which will make them waterproof indefinitely.

L. J. KLOTZ is head of the division of plant pathology if the University of California at Riverside.

J. F. L. CHILDS is a pathologist at the Subtropical Fruit Field Station of the Bureau of Plant Industry, Soils, and Agricultural Engineering, Orlando, Fla. He has degrees from the University of California.

Conidia borne in a pycnidium.