PNITEB8ITY OF C ALIFORM I A PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA Methods of Harvesting and Irrigation in Relation to Moldy Walnuts BY L. D. BATCHELOR BULLETIN No. 367 June, 1923 UNIVERSITY OF CALIFORNIA PRESS BERKELEY, CALIFORNIA 1923 David P. Barrows, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Dean. Edward J. Wickson, Horticulture (Emeritus). 4 , Director of Resident Instruction. C. M. Haring, Veterinary Science; Director of Agricultural Experiment Station. B. H. Crocheron, Director of Agricultural Extension. C. B. Hutchison, Plant Breeding; Director of the Branch of the College of Agriculture, Davis. H. J. Webber, Sub-tropical Horticulture; Director of Citrus Experiment Station. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Ralph E. Smith, Plant Pathology. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Fruit Products. Warren T. Clarke, Agricultural Extension. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. Walter Mulford, Forestry. James T. Barrett, Plant Pathology. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. L. D. Batchelor, Orchard Management. W. L. Howard, Pomology. *Frank Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. R. L. Adams, Farm Management. W. B. Herms, Entomology and Parasitology. John E. Dougherty, Poultry Husbandry. D. R. Hoagland, Plant Nutrition. G. H. Hart, Veterinary Science. L. J. Fletcher, Agricultural Engineering. Edwin C. Voorhies, Assistant to the Dean. CITRUS EXPERIMENT STATION division of orchard management L. D. Batchelor Ralph G. LaRue J. G. Surr E. R. Parker * In cooperation with Division of Agricultural Engineering, Bureau of Public Roads, U. S. Department of Agriculture. METHODS OF HARVESTING AND IRRIGATION IN RELATION TO MOLDY WALNUTS* By L. D. BATCHELORt CONTENTS page Introduction 677 Seriousness and Distribution of the Mold 677 Specific Fungi Causing Moldy Nuts 679 Appearance of Mold on Walnut Kernels 680 Entrance of Mold into Nuts 680 Occurrence of Mold in Seedling and Budded Nuts 681 Variation Among Seedling Trees 682 Extent of Variation 682 Likelihood of Individual Trees Producing Moldy Nuts Year after Year 683 Relation of Mold to Side-Blighted Nuts 683 Relation of Condition of Nuts at Harvest to Presence of Mold 684 Maturity of Nuts 684 Mold Increase as Harvest Advances 685 More Mold Among Stick-Tights than Clean Nuts 686 Relation of Delay in Harvesting to Percentage of Moldy Nuts 687 Relation of Sunburned Nuts to Prevalence of Mold 690 Relation of Curing to Percentage of Moldy Nuts 692 Relation of Irrigation to Percentage of Moldy Nuts 692 Summary .' 695 Practical Recommendations 695 INTRODUCTION SERIOUSNESS AND DISTRIBUTION OF THE MOLD For many years the prevalence of mold on walnut kernels has been one of the important causes of reduction in grade of nuts and therefore of loss of money to the walnut growers. A dark discolora- tion of the kernels is frequently, but not invariably, associated with the mold. In certain sections this trouble has been very bad the past three or four years. * Paper No. 109, University of California, Graduate School of Tropical Agri- culture and Citrus Experiment Station, Riverside, California. t The author wishes to express his sincere appreciation for the many helpful suggestions in regard to this investigation, made by Dr. J. T. Barrett, Professor R. S. Vaile and Mr. Carlyle Thorpe. The arduous task of cracking thousands of nuts to obtain data was accomplished by the cooperation of Mr. D. C. Wylie of the Field Department of the California Walnut Growers' Association. 678 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The extreme coastal districts and the hot inland regions have not been seriously troubled with mold. In the intermediate sections both the mold and the black kernels have frequently given a great deal of trouble. The hot inland valley sections may be troubled with a high per cent of black kernels even in the absence of mold. The most serious trouble with moldy nuts is usually found on sandy soils. The economic loss to the walnut grower can be realized by calling to mind the fact that a percentage of over 10 per cent of poor nuts in any lot will necessitate the sale of the whole lot as near-grades, 1 at a probable reduction of 30 to 35 per cent of the value of first- class nuts, the exact value depending upon the actual percentage of poor nuts. A further reduction in the percentage of good nuts to less than 75 per cent good, will necessitate the whole lot being sold as culls at a loss of another 30 per cent or more of the value of good nuts. The three grades of Santa Barbara soft-shell seedlings, sold by the California Walnut Growers' Association during the season of 1921- 1922, returned the following prices to the growers: The first grade ('Diamond Brand') 2 $24.50 per hundred pounds; the near-grades averaged close to $15.00 per hundred pounds, and the culls $7.00 per hundred pounds. It should not be inferred that all the nuts in the inferior classes are poor quality nuts, but rather that the percentage of poor nuts in the near-grades and culls is too large compared with the good nuts to justify their being sold as a first-class product. It should be clear from this discussion that it takes only a relatively small percentage of inferior nuts to lower the grade and the value of a lot of nuts, the majority of which are of a good quality. If a given lot of nuts is near the border line between the above grades, only one or two pounds of moldy nuts per hundred, may change the com- mercial value of the whole lot $7.00 to $8.00 per hundred pounds. Most of the moldy nuts cannot be sorted out in the packing house; thus great losses have been experienced in the past which were unavoidable by any means of grading or sorting practiced up to the present time. I'Near Grades' are an inferior commercial grade in which the good nuts comprise only 75 per cent to 89 per cent of the total. The rest are moldy, shriveled, sun-burned, and generally inedible. 2 'Diamond Brand.' is the trade name for the best grade of nuts shipped by The California Walnut Growers' Association and is guaranteed to contain at least 90 per cent perfect nuts. Bulletin 367] harvesting and irrigation of moldy walnuts 670 The efforts of the walnut growers during a period of practically a year are expended on producing high-grade nuts. Good nuts can be changed like magic into the cull class in a week or ten days of improper handling. The work herein reported has shown that the nuts which are finally graded as culls because of mold or dark kernels, were without question first-class nuts at their maturity, a period which should coincide with the beginning of the normal harvest season. SPECIFIC FUNGI CAUSING MOLDY NUTS The particular fungi which cause the mold in walnuts have been studied during the past three years by Dr. J. T. Barrett of Citrus Experiment Station, Riverside, to whom the writer is indebted for the following statements: "Mold in walnuts is not, strictly speaking, a specific disease, therefore, not necessarily confined to the action of a single specific organism. Many fungi and bacteria may grow for a time, upon matured parts of plants which offer suitable conditions of moisture and nutrient material, without actually destroying much of the tissue. The checking of the favorable conditions checks the growth of the organism. Therefore, it was no surprise to find that several fungi could cause moldy walnuts. No doubt many others would produce a similar effect were they present in the groves in sufficient quantity, and at a time when the nuts were in the proper stage for infection. "Determination of the fungi most commonly associated with the moldy nuts was by means of cultures made by transferring pieces of the mold or mold-bearing tissue of the kernel to a suitable nutrient medium, usually glucose potato-extract agar. Usually within a week to ten days sufficient growth had taken place to make the determination possible. The many cultures made represented sample collections of nuts from more than twenty-five groves distributed in twelve of the main walnut-growing sections. "The result of these studies revealed that by far the most preva- lent fungus in moldy nuts is Alternaria, a genus containing many species, some of which cause very serious diseases of plants, many others occurring as saprophytes only. A little more than seventy- six per cent of the total number of nuts cultured were infected with Alternaria. Less than one per cent showed some other fungus asso- ciated with the Alternaria. While no attempt has yet been made to determine the species, the character of growth and spores in the majority of the cultures examined indicates that one species princi- pally occurs. 080 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION "The largest number of cultures were made from nuts collected from the six districts most seriously troubled with moldy nuts. It is interesting to note that eighty-five per cent of these cultures developed Alternaria. "Several other fungi were found in moldy nuts, but not at all consistently, and in a number of cases ranging from one-half to five per cent of the total number of cultures. "The genera represented are Penicillium (blue and green mold), Cladosporium, Fusarium, Macrosporium (very similar to Alternaria), Mucor, and Sclerotinia (form with small sclerotia). About five per cent of the cultures were negative, and two per cent developed fungi that were sterile and not determined. "While these studies to date are of a rather preliminary character, the evidence is sufficient to indicate very strongly that the fungus genus most responsible for the moldy nuts, during the past three years is Alternaria. How many species are concerned has not been determined. This with a number of other phases of the problem is to be further studied." APPEARANCE OF MOLD ON WALNUT KERNELS In the worst cases of mold the abundance of fungous growth, known as mycelium, is seen as a light to grayish-colored cottony growth on the kernels and inner walls of the shell. If the growth is only slight and largely confined to the diaphragm of the shell, it may not be detected. On the other hand, by the aid of a magni- fying glass, strands of fungous mycelium may be detected on the kernel of many first-grade nuts, indicating that under suitable condi- tion most nuts would probably be subject to infection. On the drying of the nuts the mold dries also, but does not by any means disappear. It retains its fluffy appearance but may collapse somewhat on removal of the shell. There is not a marked difference in appearance of most of the various mold fungi as seen on the kernel. Sclerotinia and Fusarium are usually distinguishable from Alternaria, Macrosporium, and Cladosporium, although one may be frequently mistaken. ENTRANCE OF MOLD INTO NUTS Among the many thousand nuts which have been cracked and examined in connection with these studies, no mold has been found in a nut which at the time of examination was incased in a sound husk free from visible cracks and in crisp condition free from BULLETIN 367] HARVESTING AND IRRIGATION OF MOLDY WALNUTS 681 decay. The mold commonly starts to grow in the cracks of the husk which accompany maturity, and spreads thence to the lining of the husk; it quickly spreads through the base and suture of the nut and thence to the diaphragm, and the pellicle of the kernel. Nuts frequently drop to the ground with the green husk intact. If such nuts lie on the ground a few days the husks begin to decay. Alternaria is the most common cause of this decay. A soft and decayed spot usually first appears on the apex of the shuck. In such cases the mold spreads to the kernel by passing through the suture at apex of the nut. Even in the worst cases of mold the fungus is superficial on the pellicle and does not actually affect the eating qualities of the kernel except as it presents an inedible appearance. OCCUEEENCE OF MOLD IN SEEDLING AND BUDDED NUTS The trouble with mold has been much worse among the seedling groves than in the 'budded' 3 ones. This difference is mainly due to the fact that the budded varieties are more apt to drop the nuts from the tree free from the husk. Many of the old seedling trees are characterized by a high per- centage of the nuts dropping as green 'stick-tights.' 4 The seedlings which more nearly approach the paper-shell type are more likely to shed the nuts as green stickers than the typical Santa Barbara soft-shell type. The budded varieties, especially of the Santa Barbara soft-shell type such as Placentia and Chase, are, however, not entirely free from this trouble. Moldy nuts have occurred among both of these varieties to a serious degree, when for one reason or another the conditions have been favorable to mold development. The fact that the nuts of budded varieties usually drop free from the shucks is not the only factor which lessens the likelihood of mold among this class of nuts. Another reason is the fact that the majority of the budded groves are younger than the seedling groves, and for that reason are not as apt to become drought-stricken during the growing season as the older trees. As will be shown later, there is a correlation between drought and mold. s The term 'budded' walnut tree is a trade name to designate the most desirable commercial varieties of nuts which may be propagated by budding or grafting them on black walnut roots. 4 The term ' stick-tight' has become universally used by walnut growers to designate the nuts which drop to the ground with the husks sticking to them. 682 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION VARIATION AMONG SEEDLING TREES EXTENT OF VARIATION A group of eighty seedling trees have been under observation for the past three years. The prevalence of moldy nuts produced by the respective trees has varied greatly each year. The range and mean per cent of moldy nuts per tree are shown in table 1. At least fifty 5 nuts per tree were examined in each sample. TABLE 1 Range of Variation and Mean Per Cent of Mold Found in Nuts from a Group of Seedling Trees Total per cent of mold in clean nuts Year Range among individual trees Mean of all trees 1920 0—56 0—24 0—48 16 1921 7 1922 11 It is very apparent from table 1 that a group of seedling trees is an extremely variable population as regards this character of susceptibility of the nuts to mold. One tree may produce nuts which are free from mold, while possibly the adjacent tree produces 20 to 50 per cent moldy. Some of the wide variation is probably due to the inherent tendency of certain trees to produce stick-tight nuts, which are more apt to mold than clean nuts. Some of the apparent variation in the percentage of moldy nuts from different trees may be due to the impracticability of harvest- ing the crop from each seedling tree when the nuts are in exactly the same state of maturity. Again the variation in the soil and subsoil makes it impossible to have the available moisture in the root zone of each individual tree alike. Such a variation in moisture is bound to be reflected in maturity of the crop, especially in the ease with which the nuts are husked, with a consequent prevalence of mo 1 (I. From a practical point of view, it is important to know that some particular trees are much more likely to produce moldj* nuts than others, as it may become advisable to segregate the moldy nuts in the orchard. 5 It was found by experience that a sample of fifty nuts gave practically as accurate results as one hundred or more, provided the respective lot from which the sample was drawn was properly mixed, so the laws of chance came into fair play. BULLETIN 367] HARVESTING AND IRRIGATION OF MOLDY WALNUTS 683 LIKELIHOOD OF INDIVIDUAL TREES PEODUCING MOLDY NUTS YEAE AFTEE YEAE The probability that certain trees may produce a large per cent of moldy nuts year after year has been shown to only a small degree by the studies of the past three years. Even though there is a small correlation between the percentage of moldy nuts from individual trees, comparing one year with another, there are many exceptions to this general rule. A strict interpretation of the statistical data 6 at hand not only shows the uncertainty of foretelling the relative amount of mold from individual trees in the future from records of the past; but the harvesting and curing experiments herein set forth show that these last mentioned factors are largely responsible for the prevalence of favorable mold-producing conditions. RELATION TO SIDE-BLIGHTED NUTS Inasmuch as the mold organism usually enters the husk after it starts to crack, it was thought that the mold might enter the small fissures in the husk caused by the form of walnut blight commonly known as side-blight. Many nuts which are side-blighted to a slight extent mature as good nuts, even though the husk may be affected with blight. Such blight cankers may not fully penetrate the husk sufficiently to stain the shell of the nut, even though the husk is clearly ruptured by the blight. During the middle of the 1922 growing season, a large number of side-blighted nuts were covered with gauze bags so the nuts could be identified at harvest time. Likewise an equal number of healthy nuts were covered. Upon examination at harvest time the blighted nuts showed a slightly higher per cent of moldy nuts than the healthy nuts. The difference was more marked among stick-tight nuts than those which dropped free from the husks. A larger per cent of the side-blighted nuts were finally stick-tights than in the case of the healthy nuts. 6 A detailed presentation of the statistical data is not called for at this ttme. It is sufficient to state here the correlation coefficients for the per cent of moldy nuts per individual tree in both clean-nut and stick-tight classes were calculated, comparing the following harvest seasons: 1920 with 1921; 1920 with 1922; 1921 with 1922. Although all six coefficients were positive, the low average of only .33 with a probable error of .07 shows only a low degree of probability that a tree will produce markedly moldy nuts year after year, other factors being equal. 684 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Although, there is a positive association of side-blight and mold, it is doubtful if this is a very important factor. Where there is a large enough percentage of mold to be a serious commercial con- sideration, the majority of cases become infected by other means than through blight cankers in the husks. RELATION OF CONDITION OF NUTS AT HARVEST TO PRESENCE OF MOLD MATUEITY OF NUTS The relation of the mold to the progress of the ripening of the nuts was suggested from the records of 1920. This investigation also showed a possible relation of harvesting methods to moldy nuts. Twelve thousand nuts were cracked to obtain this test, relative to the relation of harvest methods to moldy nuts. The nuts were equally divided into three lots: First, nuts which were picked from the trees when matured so the husks were starting to crack; second, clean nuts from ground; and third, stick-tights, the husks of some of which were black and dry when they fell, while others were originally mushy but had lain around until the husks were dried up. The percentage of mold in these three classes is shown in table 2. TABLE 2 Progress of Mold Development in Nuts at Different Stages of Maturing, and Conditions at the Same Date of Harvest (1920 Crop) Classification Moldy culls Moldy but passable Total mold Nuts picked from trees when husks started to crack 6% 4% 10% Clean nuts from the ground 7%' 8% 15% Black stick-tight nuts from ground 13% 14% ■ 27%. • :. The moldy nuts were divided into two classes according to the amount of mold present: (a) moldy culls; (b) moldy but passable. Tn the latter class the mold was clearly visible on the kernels, but not bad enough to condemn them as culls. At the same time there was a certain additional percentage of nuts which showed only such small traces of mold that they were classed as first-class nuts. BULLETIN 367] HARVESTING AND IRRIGATION OF MOLDY WALNUTS 685 It is clear from a study of the summary that a delay in harvest- ing is bound to permit the mold to increase. Even the nuts picked from the trees were 10 per cent moldy. As some of this mold was passable, however, it would not have barred them from the best grade if they had all been periodically harvested as soon as the husks cracked. Many of the nuts in the lots picked off the trees had been held in the clasp of the partly opened husks for several days. The prompt harvesting of the nuts would have prevented the develop- ment of the mold, and have made the whole crop grade as Diamond Brand. A delay of several weeks would have allowed the whole crop to become so moldy that it would have graded as near-grades or culls. This point is brought in again in several of the following summaries : MOLD INCREASE AS HARVEST SEASON ADVANCES (Harvested according to ordinary methods) Harvest and cracking records were kept from a group of trees near Anaheim during 1922. The nuts were picked up three or four times between September 28 and November 1. The trees were shaken each time, except at the first picking. One hundred nuts per tree were cracked at each picking to determine the amount of mold in the clean and stick-tight nuts. Table 3 shows the increase in the percentage of mold as the harvest season advanced. TABLE 3 Increase in Percentage of Moldy Nuts as the Harvest Season Advanced , 1st pick (Sept. 23-28) 2d pick (Oct. 10-14) 3d pick (Oct. 25-Nov. 1) Moldy culls Moldy but passable Moldy culls Moldy but passable Moldy culls Moldy but passable Clean nuts 3% 9% 6% 16% 5% 4% 12% 23% 9% 18% 27% 4% 15% 10% Dry stick-tights 28 % Green stick-tights , Table 3 . as well as table 2 shows that the dry stick-tight nuts are much more likely to be moldy than clean nuts. This is again brought out in table 4 which shows a summary of the records of all three years. The main purpose of table 3, how- ever, is to show the increase in percentage of moldy nuts as the season advances. 686 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION With the clean nuts and the black, dry stick-tights there was an increase in moldy nuts, comparing the first pick with the second one. Still further increase of mold was noted in the black stick- tights between the second and third picks. This big increase in mold is bad enough in the clean nuts and black stick-tights, but an even greater increase is shown by the green stick-tights. This last mentioned class of nuts was practically free from moldy culls at the first picking. Twelve to fourteen days later, when the trees were shaken for the first time, the green stick-tights were 50 per cent moldy with 23 out of every 100 nuts reduced to moldy culls. Some of the green stick-tights of this second pick were already on the ground before the trees were shaken. Possibly 50 per cent of them were shaken off. It is interesting to note what 100 pounds of the green stick-tights were worth at the first pick compared with the second one. With such nuts originally grading as Diamond Brand when properly harvested, they were worth $22.50 per 100 pounds. Although there were 50 pounds of good nuts in every 100 pounds of the second pick, they could not be sorted out of the near-grades and culls. At the very best the whole lot could scarcely be counted on to grade as near-grades, valued at $15.00 or less per 100 pounds. A few more days delay in harvesting would, no doubt, have made culls of them, worth possibly $7.50. It is only a matter of combining plain arithmetic and good judgment to see that it would pay big dividends to speed up the harvest of green stick-tights, even to the extent of spending fifty cents per 100 pounds or more as an extra bonus for harvesting and shucking this type of nuts. It would also require, no doubt, more personal supervision of the laborers by the grove owner, which is a good thing on general principles, and would be well paid for in such a case as above illustrated. MORE MOLD AMONG STICK-TIGHTS THAN CLEAN NUTS In tables 2 and 3 it has already been shown that stick-tights are more apt to be moldy than are clean nuts. The following table summarizes the observations for all three years. With the large number of nuts observed (28,451) to make this comparison, the small errors of sampling, and the personal errors of judgment have been leveled by the laws of chance. It becomes a practical certainty that stick-tight nuts at harvest are likely to include somewhat more than twice as many moldy nuts than are the clean-shelled nuts. bJO o • t fmmm Jf 4 o s © a e o © H-i so Ji ^ ^ xi +j en 3 P «> -° S 3 o "IT" V S B *» «> a ■g +-> u o o - a o3 1 2 £ be o t5 Zl p >> o XI CO P d s o T3 o a o CO M '<£ o 03 © 3 p >-, © s CO o o bJD