Injury to the leaves resulting from attacks by diseases and insects is one of the most common and important causes of poorly filled nuts. Every species and variety of tree nut suffers from at least one disease or insect pest that damages the leaves and hence limits or curtails the amount of elaborated food materials they can make. In most cases the fungi or bacteria causing foliage diseases infect the leaves in early spring at the time they are unfolding and growing in size, although the infection may not be noticeable until later. These infected areas, even though they are small and not numerous enough to cause the leaf to fall, seriously impair the functioning of the leaf out of all proportion to the area directly affected. Should the infection be so severe as to cause premature defoliation, the damage will be great even though only a small percentage of the leaves falls. The disease of eastern Mack walnut known as leaf spot, or anthracnose, is one of these defoliating diseases that causes untold damage from poorly filled nuts in the current crop year, and results in a small crop or none at all the following year. The development and spread of these diseases is gradual, and unsuspecting growers do not realize the damage they cause.
On other hand, the injuries caused by such insects as the webworm, the walnut caterpillar, the pecan leaf case-bearer, the Japanese beetle, and others are somewhat spectacular in that the leaves may be partly or completely consumed on portions of the trees. The injury caused by the walnut aphis, the walnut lace bug, the pecan black aphis, and others, on the other hand, is less conspicuous; but the end result is far more serious than it usually is with the leaf eating insects, because the damage caused is more widespread, almost all of the leaves on a tree being affected. These sucking insects are small in size and may be overlooked until premature defoliation takes place. If nut trees are to bear satisfactory crops of well filled nuts, the diseases and insects that attack and cause injury to the leaves must be controlled. Under normal conditions the size of the crop produced, the regularity of bearing, and the quality of the nuts harvested is proportional to the leaf area of normal leaves carried by the tree from early spring until freezing-weather in the fall.
+Second growth of the trees:+ Certain of our nut trees, such as pecan and walnuts, under some conditions have two or perhaps more periods of shoot growth during the same growing season. The first, or main period of growth, starts at the time of foliation in the spring and ends soon after the shoots flower. The second period of growth, if it occurs, may begin any time after the nuts are set, and may end any time later. This second growth seriously affects the filling of the nuts, in that food materials are consumed in producing this second growth rather than in the growth and filling of the nuts. Generally this second growth is not made until late in the season, and it usually follows a period of dry weather, when conditions again become favorable for growth. Usually this is at the time the kernels should be developing, and hence the degree of filling is affected. The seriousness of the effect on the filling of the nuts is largely proportional to the amount and duration of this second growth. A third period of growth may occur later if weather conditions are suitable.
+Preceding crop:+ It has already been pointed out that nuts are storage organs and in their growth and development large amounts of food materials and minerals are removed from the tree. Under conditions of heavy crop production, the reserves of these materials left in the tree at the time of harvest are likely to be very low; and unless the trees are growing on a fertile soil and carry their leaves until frost, these reserves of minerals and elaborated food materials are not likely to be restored. Under such conditions, in the following spring the reserves are low and although there may be enough to initiate flowering and the set of nuts, they are not sufficiently high to produce well filled nuts. It is for this reason that the nuts produced in an "off crop year," even though the crop may be much lighter, may be less well filled than those produced in an "on crop year."
Such nuts as pecans, hickory nuts, and walnuts transfer large amounts of potassium from the tree itself into the shucks or hulls. The kernels of such nuts are high in nitrogen, phosphorus, and magnesium as well as in oil, which is one of the most concentrated food materials and has the highest calorie value. Nitrogen reserves in the trees are readily and rather quickly replaced if adequate amounts are applied, as this element is not fixed by the soil. This is not true of phosphorus and potassium, as they are apparently taken up by the trees much more slowly than is nitrogen. Furthermore, certain soils have a high fixing power for these elements and hence they are slowly, if at all, available.
+Insect and disease damage to the nuts:+ Certain insects and diseases attack the nuts, causing them to be poorly filled at harvest. Although these pests may destroy, or cause a certain percentage of the crop to drop before harvest, and hence serve as a thinning measure, the affected nuts remaining on the tree may not be well filled at maturity. Examples of such insects are the pecan or hickory shuck worm, the walnut husk maggot, and the codling moth. Infestations by these insects occurring before the shells of the nuts have become hard cause the nuts to drop. However, infestations taking place after the nut shells have become hard do not cause the nuts to drop. These late-infested nuts may be poorly filled because the insect larvae mine the hulls or shucks, severing the conducting tissues that transport food materials from the fruit stem or peduncle through the shuck to the kernel. The damage caused not only results in poorly filled nuts but also interferes with the natural separation of the shucks or hulls from the shells.
Examples of diseases that attack the nuts and cause them to be poorly filled at harvest are pecan scab and walnut bacteriosis. Pecan scab may also attack other species of hickory. It is the most destructive pecan disease, causing a high percentage of the nuts on highly susceptible varieties to drop prematurely and those that stick to the tree to be poorly filled at harvest. Walnut bacteriosis or blight is the most important walnut disease in the West and unless controlled causes severe losses from premature drop or from nuts both poorly filled and having discolored kernels at harvest. It is obvious that if large crops of well filled nuts are to be produced, these insects and diseases must be controlled.
+Weather conditions:+ Many growers are inclined to blame the weather for all small crops and poor nut quality because they realize it can have such important effects. In reality its direct effects are generally much less than they are thought to be, and its indirect effects are usually much greater than is usually realized. Weather conditions have a very great effect on the development of insects and diseases and on the damage caused by them, so that most often these are of major importance.
It has already been pointed out that a prolonged drought may adversely affect the size of nuts when it occurs while they are growing in size. Similarly, the degree to which nuts are filled at harvest is affected by the moisture supply during the filling period. A moisture deficiency within the tree probably affects the translocation of food materials to the nuts to a greater extent than it affects leaf functioning, for under such conditions the leaves will withdraw so much water from the developing nuts that the shucks and hulls become wilted. Under conditions of prolonged drought the kernels do not fill properly, maturity of the nuts is delayed, and the shucks or hulls do not open normally.
Under drought conditions the temperatures of the air and of surfaces exposed to the sun are often very high, and this sometimes results in sun-scald or burning of the hulls or shucks. In severe cases the injury extends through the hull or shuck to the shell and kernels. The pellicle, or skin of the kernel, turns brown or amber color, as does the portion of the kernel that has developed at the time of injury. Further development of the affected portion of the kernel is arrested; and on drying it becomes shriveled because of lack of filling. The greatest amount of damage from sunburn occurs on the south and southwest sides of the trees. Little can be done to prevent this type of injury other than to grow good, strong, vigorous trees that bear a heavy dense foliage that shades the nuts.