The performance of grafted Persian walnuts in southwestern Michigan has been so satisfactory that I would not hesitate to recommend them, in preference to grafted black walnuts. One of the nicest things about grafted Persian walnuts is that when they start to produce nuts, they bear every year—there is not an off-season, as with the black walnut. Our locality may be especially suitable to them. Our skies are cloudy, and it is cool through much of the spring, thus preventing early growth before conditions are right for the buds to develop unhampered by late spring frosts. We have had an occasional late freeze that caused the lower nuts to drop, while the higher ones remained on the tree, unharmed.
In this article I would like to answer briefly our most often asked question, as to which varieties do we think best from our experience with them? Our climate must be quite different from that found around Ithaca, New York, because we have never had winter injury in certain Persian varieties, as occurs in that area. (And we had 26 below zero in February, 1949.) An instance of this difference is in regard to the McDermid variety, which happens to be our choice. We honestly believe the Crath No. 1 variety to have great commercial possibilities, because of its heavy production of large, thin-shelled nuts, of average quality. The Broadview is another. The Carpathian "D", apparently, pollinates the Crath No. 1 well. This one, however, is small, with a very white kernel that is sweet. We have many other varieties producing, some with their first crop this year; but we are not able to recommend any of them yet.
The black walnut varieties must be rather limited, because of the brooming disease trouble; so we select from those that are quite able to resist it, or that seem immune to the trouble. The Thomas and Grundy varieties lead with us, and two other local nuts, the Adams and the Climax, rate high in our estimation. We have some nice grafts of the Homeland bearing their third crop, which we like very much, and they appear disease free. The Elmer Myers, Michigan, and other varieties are now badly affected with brooming disease.
Several years ago I reported on my observations on the brooming disease. Now, I wish to report a little more upon the subject, especially in regard to how certain varieties have withstood its ravages. I hesitate to make any estimation as to how prevalent the disease is in the wild black walnut today, for it could be quite a controversial subject, with some claiming I was very wrong. Anyway, many of our native walnuts are now affected. Outward appearances are often very deceiving; but, when one cuts the top off a seedling and attempts to graft it, he may be amazed at the broomy growth that soon appears from the stock, should his graft fail to take. Trees that appear healthy, but have made slow or poor growth are often affected. Short, twiggy, upright growth that soon becomes dead or partly so, and arises from the main framework of an apparently healthy tree, is one of the signs that disease is there.
I have claimed there are two, or possibly, three forms of brooming disease, and I am still as convinced as ever. The so-called "witches-broom," as commonly seen in the Japanese walnut, is the form most people seem to think of. The second form is the rapid-growing type, that lops, or arches downward, is gray or green in color of wood, is very brittle and easily broken in the wind, ripping off good sized limbs, and winter-injures badly. An investigation, will, however, show much dead wood comes before severe weather. This form has some broomy, upright growth, like the first, but it is never bunched. The other, or possibly, the third form, is the latent type that doesn't seem to do much harm, merely causing poorly filled nuts. The latent form is difficult to note, and can be detected only by the many short, dead, or partly dead, upright twigs scattered along the main framework of older trees. Cutting off part of the top will cause the typical growth to arise, thus identifying itself.
Early observation showed that certain walnut varieties were almost unaffected, or could even be immune, to the brooming disease. Different limbs of a large tree were topworked to the Thomas and the Allen varieties of black walnut. The Allen "took" the disease at once, while the Thomas grew thriftily and has always produced good crops of nuts. Later, the Calhoun variety was grafted on some lower limbs, and has remained healthy. The diseased Allen grafts are still in the tree, are now 15 years old, and are more or less alive, but in very poor condition, with the signs as found in what I call the latent form. In 1938, the McDermid Persian walnut was grafted into this same tree, and its grafts produced good crops of nuts.
I wish to cite another instance of how little the Persian walnut is affected, regardless of variety. In 1938 a large black walnut near the house was grafted with Persian grafts, on stubs that had failed the previous year. The tree had the second, or rapid growing form, of brooming disease. I have pictures showing how badly the 1938 grafts took the rapid growing form of growth; while two 1937 Persian grafts showed no signs of trouble. The tree started to bear in 1941, and has made remarkable growth. It is now one of the nicest Persian walnut trees I have, bearing heavily every year. It is about 30 feet tall and 20 feet broad, with no apparent signs that it was ever affected.
I feel we should recognize the fact that eradication of brooming disease is impossible; but one should plant, or graft, those varieties known to bear good crops in spite of this trouble. The Thomas and Grundy black walnuts do very well here, as well as the two local nuts mentioned. I do not know of any Persian varieties affected. I do not have any Persian trees with the typical broomy bunch, as is so often seen in the Japanese walnut, and its hybrids. The native black walnuts, when affected, seem to fail to fill properly, are immature, and watery, black veined, and worthless at harvest time, shriveling to a dark, hard, kernel when cured.
I think this answers the oft-asked question, "Why do not my black walnuts fill as they used to?" There is a strange relation to the filling of the native black walnut and the days of 1934 and 1935, when we had the great walnut caterpillar scourge!—when the trees were stripped of all their leaves. Ever since, we have had the brooming disease to contend with. One could jump to the conclusion that improper filling and this trouble were caused by a lack of certain nutrients; but seedlings in nursery rows are often affected, even where they are given every care.
At one time this spring I thought I had found a new way of "bench-grafting" walnuts. Seven grafts, on black root, were made in December, and were planted directly in a frost-proof coldframe, as lilacs can be grafted. All seven grafts made good growth, that is, over three inches, by early May, but failed later. There is only one alive today, I do not think this an impossible method, but there must be a better way of handling to give success, such as attention to shading and careful watering. One may find more on this subject in "Propagation of Trees, Shrubs, and Conifers," by Wilfrid G. Sheat.