+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.