Seeds for Rootstocks of Fruit and Nut Trees

L. C. COCHRAN, W. C. COOPER, AND EARLE C. BLODGETT.

THE PRINCIPAL fruit and nut trees grown commercially in the United States (except figs, tung, and filberts) are grown as varieties or clonal lines propagated on rootstocks.

Almost all the rootstocks are grown from seed. The resulting seedlings then are either budded or grafted with propagating wood of the desired variety. This practice has come about chiefly because the improved varieties of these fruits and nuts do not come true from seed and are not easily propagated on their own roots from cuttings.

Seedlings used for rootstocks generally are easy to grow. Experience has taught us which scion-rootstock combinations are suitable and how to produce good nursery trees.

Growing fruit and nut trees on rootstocks rather than on their own roots has many advantages.

Some of the desirable horticultural varieties have more vigor when they are grown on vigorous rootstocks. Some varieties on their own roots are susceptible to root-infecting, disease-causing organisms and nematodes, root- and crown-infesting insects. Some are intolerant to salt, drought, alkali, and poorly drained soils. The cold hardiness of some varieties can be increased by top-working them onto the framework of cold-hardy varieties.

Many varieties produce fruit of better quality and are more productive on suitable rootstocks than they are on their own roots. By a choice of rootstocks, some fruit and nut crops can be grown in areas otherwise not suitable.

In instances where rootstocks have solved problems for one crop, their use has led to calamity in another.

An example is the sour orange. Because of its adaptability to heavy soils, resistance to foot rot, relatively high resistance to cold, and production of good crops, the sour orange has become widely used throughout the world as a rootstock for sweet oranges.

The tristeza virus, which destroys sweet orange trees growing on sour orange roots, got a foothold in South America. It spread and killed millions of trees and almost wiped out the industry. The tristeza virus has now become universal there, and the industry is being reestablished only by the use of tolerant rootstocks, such as Cleopatra mandarin, sweet orange, Rangpur lime, and rough lemon.

Many factors, some of which are changing continuously, determine the suitability of rootstocks. Likewise, new information is being developed constantly, and new and better rootstocks become available to meet the need.

A GOOD ROOTSTOCK is one on which the desired variety of fruit or nut makes a good graft union and on which it is long lived, yields well, grows relatively fast, and fruits early. It must be reasonably compatible with the top variety.

If the rootstock variety is vigorous and the top variety is thoroughly compatible on it, the resulting tree tends to grow fast, is large, and often comes into fruit late. Such over-vigorous combinations sometimes result in reduced quality or yield of fruit.

On the other hand, when the rootstock and top are not thoroughly compatible, the tree is likely to be dwarfed. Such incompatibility is often the result of failure of suitable formation or function of the phloem at or near the bud union. The phloem is the tissue that contains the food-conducting tubes in the bark through which the elaborated food made in the leaves flows to the roots to nourish them. Because of reduced nutrition, the roots are dwarfed, and there is a corresponding dwarfing of the tops.

The reduced function of the phloem at the bud union leads to an accumulation of elaborated food in the tops, that in turn causes earlier maturation of fruiting wood and earlier fruiting.

Nurserymen have made use of such partial incompatibility directly or with interstocks to produce dwarf trees, which fruit correspondingly at an earlier age.

Other desirable features claimed for dwarfed trees are cheaper and easier control of pests; lower cost of pruning, thinning, and harvesting; and higher quality and better color of fruit.

The problem in producing dwarf trees is to find combinations that allow enough of the normal functions to go on so that the trees will have the desired size; not be subject to winter injury, malnutrition, and disease; and be productive of good fruit.

A ROOTSTOCK seed parent should produce seedlings that are uniform in size, vigor, and the qualities which make a seedling a good rootstock.

Preferably a rootstock seed parent should be self-fertile so that it can be grown in isolated blocks and thereby prevent hybridization and variability in the seedlings. The fruits and seeds should all mature at the same season to permit machine harvesting. If the seeds are in fleshy fruits (such as cherries and plums), the fruit should be a freestone to allow easier removal of the fruit flesh.

The trees that are sources of seed should be productive. The seeds should give a high percentage of germination.

THE SEEDS of fruit-tree rootstocks have to be harvested, stored, and handled in different ways.

Some cannot be dried or allowed to ferment in the juice of the fruit. Some need afterripening before they will germinate. Some remain viable in a dry condition for several years.

Because citrus seed soon loses its vitality if it becomes too dry, it is usually extracted from fresh fruit by hand, washed free of pulp and juice, surface dried, and planted immediately. If it is to be stored, the washed seed should be dipped in a 1-percent solution of 8-oxyquinoline sulfate, surface dried, and placed in a polyethylene plastic bag and kept in cold storage at 38 - 40 F. Such treated citrus seed can be stored many months with only a slight decline in viability.

SEEDS of peach, apricot, cherry, and plum should be extracted from fresh fruit and preferably washed free of fruit parts and juice as soon as possible. These stone-fruit seeds lose their ability to germinate if they are allowed to ferment in fruit juice or fruit pomace. Viability is severely reduced if left in fermenting juice even for 24 hours. On the other hand, properly dried and cured seeds of peach, plums, and apricot may be kept 4 years or more in a cool, dry storage with little loss in viability.

Cherry seeds generally are more sensitive to drying. After harvest they should be thoroughly washed, surface dried, and kept in a cool, moist storage until they are ready for planting or placed in storage for afterripening. In some sections, cherry root- stock seeds are planted directly in the nursery in the fall. Some growers prefer to hold them in a mixture of moist peat and sand in a cold storage and plant them in the spring. Cherries need approximately 110 to 120 days at 40 for best germination.

Mazzard and mahaleb cherries are the commonest rootstocks for sweet cherries. Some cherries are grown exclusively on mahaleb. Seeds of both mahaleb and mazzard do not remain viable as long as peaches and apricots.

Seeds of apple, pear, and quince mainly are harvested from local varieties at processing plants, washed, surface dried, and stored in cool, moist storage. Sometimes they are planted immediately directly in the nursery row.

Almonds can be air dried. They will retain their viability for several years in common dry storage.

pecan seeds are harvested and planted immediately in the fall or they can be stratified in moist sand until they germinate when they are planted in the nursery row.

No entirely satisfactory rootstock has been found for the Regia or Persian walnuts. Yuglans hindsii, the northern California black walnut, Regia seedlings, and Royal and Paradox hybrids are all used. The seed is stored over winter in a cool, moist place and planted in the spring.

MANY TREE seeds need a rest or afterripening period before they will germinate.

Generally speaking, seeds with a hard shell or pit require more rest than those with a softer shell or without a shell.

Most soft or fleshy seeds that do not stand drying usually will grow almost immediately as soon as they are put in a suitable environment for germination.

Most seeds acquire their needed rest period or chilling requirement only when placed under proper environment. Most varieties of peach need 100 to 120 days in a moist substrate, such as a mixture of moist peat and sand at 40 , to give maximum germination.

A few seeds will germinate after 60 days, and the percentage increases with time. Peach seeds can be held at 32 for several months after their rest period has been satisfied without germination, but will germinate immediately if the temperature is raised.

Moisture is as important as cold in the afterripening process. Most deciduous seeds in temperate climates will receive more rest than they need if they are planted directly in the nursery row and allowed to obtain their chilling from natural exposure. Freezing is not necessary but usually is not harmful unless it is severe.

Apricots, almonds, cherries, and plums can be afterripened in the same way as peaches.

Some plums, especially domestics species, require much more chilling than most peaches. The myrobalan plum, Prunus cerasifera, requires about the same chilling as peaches.

Mahaleb cherries require slightly more than peaches, up to 125 days, and some mazzard cherries may require even more.

Apricots and almonds require only 50 to 60 days for after-ripening.

Pears and apples require only 6 weeks or less.

In general, the seeds cannot be dried after they have had their rest period without losing their viability.

Seeds of some fruits, such as peaches and cherries, will continue to grow if removed from fruits at maturity and placed in a proper environment without drying. No chilling is needed for such seed unless they are allowed to stop growing, in which case they will require chilling before growth is resumed.

FEW DISEASES are spread in or on seeds used for rootstocks. Most important are virus diseases.

No viruses are known to be carried in citrus seeds, although most of the commercial citrus trees throughout the world are infected with virus. Some of the viruses, like psorosis, produce violent and devastating symptoms. Others are latent unless the variety is top-worked on a sensitive rootstock. Citrus varieties and species that produce asexual seeds can be reconstituted free of virus by merely growing seedlings, which are propagated.