CONTROL OF PECAN SCAB DURING THE WINTER MONTHS RICHARD H. CONVERSE¹ Summary Orchard sanitation and dormant sprays of lime-sulfur and Bordeaux mixture have not controlled pecan scab. Puratized Agricultural Spray, Santobrite, or Corona CM-220, applied to dormant pecan trees and to the ground beneath them and followed by the regular summer spray program of 5 Zerlate spray applications, gave much better control of scab than did 5 Zerlate sprays alone in Oklahoma tests. Dormant sprays alone, however, gave poorer control than the Zerlate sprays alone. The 5 Zerlate applications were each rated for their value in the schedule. The earliest spray (half leaf) was found not to be necessary. A very close relation was found between the amount of scab on a given tree in one year and the amount on the same tree the next year. The lower the scab level on a tree in one year the lower it was the next year. In any one year, however, disregarding carryover effects, a moderate amount of scab (one-fifth of the shuck surface scabbed in September) could be tolerated without seriously decreasing quality of nuts. A minimum spray program in Oklahoma for scab control in ungraced orchards would consist of 1-2 dormant sprays plus 3-4 summer sprays of Zerlate. The wetter the year the higher the number of sprays ___________ ¹Agent (Plant Pathologist) Oklahoma Agricultural Experiment Station, and the U.S.D.A., A.R.S., C.R.D. cooperating, Stillwater, Oklahoma. required. Because of the carryover of scab from year to year a spray schedule should be followed that will aim at complete scab control each year. Dormant sprays are not safe to apply in grazed orchards at present. The common method of controlling pecan scab at present is the use of fungicide applications to protect the susceptible parts of the tree during the growing season. Zerlate, Dithane Z-78 and other materials² are used in schedules that may involve as many as 6 spray applications during the growing season. In addition to protecting the pecan from infection by scab spores another type of control is the eradication of the holdover sources’ of these spores before the tree leafs out in the spring. Both mechanical and chemical means of dormant-season eradication have been tried. J. B. Demaree and others have suggested cultural practices (disking under old nut shucks and leaves in late winter) to lower the level of early spring infection sources. Scabby twigs, nut shucks, and peduncles (the part of the stem to which the nuts are attached) that remain in the tree cannot be plowed under, however, and are too numerous to prune out. These sources of the scab pathogen are the most important for causing scab infections in the spring and are best eliminated by dormant spraying. Strong Bordeaux mixture was used by J. B. Demaree, J. R. Cole, J. R. Large, and A. M. Phillips in dormant spray applications without success. Lime-sulfur and Elgetol have also been used with no improvement in scab control or with inconclusive results. In a series of experiments from 1953 to 1956, a number of materials were tested as possible contact (eradicant) fungicides for the control of pecan scab during the dormant season. Out of a large number of compounds screened in the laboratory and in pilot field tests Puratized Agricultural Spray, Santobrite3, and Corona CM-220 showed promise and were used in full-scale field tests. Dormant spray applications of these materials were made to pecan tree (Squirrel variety) and to the ground beneath them out to the branch-drip line a few weeks before bud break with a standard high-pressure hydraulic sprayer and hand gun. Care was taken to cover the trees thoroughly and to soak the ground beneath them. To insure as complete eradication of holdover sources _________ ²To simplify the information in this paper trade names of fungicides have been used. No endorsement of the named products is intended nor is criticism implied of similar products which are not mentioned. ³See Table 1, footnote 2. of the scab as possible, 2 separate applications were made in some experiments. In one case a row-crop spray boom was used to drench the ground in a block of 5 trees and then the trees themselves were thoroughly sprayed. Representative data from 2 field tests (Table 1) show the effectiveness of dormant spray applications of contact fungicides in reducing the source of early spring infections and keeping scab from developing on the pecan nuts. It was soon learned that 1 or even 2 dormant sprays of these fungicides alone will not control scab satisfactorily. However, the addition of a single dormant spray application of Corona CM-220 to an application schedule of 5 Zerlate sprays resulted in a 55 percent reduction in scab on the nuts the following September as compared to the Zerlate schedule alone. When all the ground in a block of dormant trees was sprayed with Corona CM-220 and the block then received the standard summer spray schedule, scab was at the trace level at the end of the season. Spraying to control scab is expensive, representing about one-third of the total cost of production, and no grower wants to spray more than will pay off. We have examined the Oklahoma summer spray schedule to see what a minimum safe schedule might be, (Table 2). Our regular scab schedule in Oklahoma during the growing season past consists of 5 applications: 1) Half leaf, 2) Case bearer, 3) Late June, 4) Mid-July, and 5) Mid-August. One additional prepollination spray has also been recommended in heavy scab years in the past. In 1953, our last heavy scab year in central Oklahoma, omission of the half-leaf application in the standard 5-pray schedule did not make a statistically significant difference in the amount of scab control. Leaving Zerlate out of the early June case bearer application resulted in a significant increase in scab as compared with the full schedule, and omission of any 1 of the last 3 applications resulted in high scab levels. It was repeatedly noticed that dormant-sprayed trees or trees having few twig lesions and scabby shucks still in the tree remained scab-free or nearly so well into the summer, even though adjoining unsprayed trees were badly scabbed. A number of laboratory and field tests hare indicated that scab spores are not readily carried by dry winds although they are readily spread by water (in the form of rain drops and wind-driven rain). When a number of Squirrel pecan trees that had been given various spray treatments in 1955 were left unsprayed in 1956, it was found that the amount of scab developing on any particular tree in 1956 was very closely related to the amount of scab on that same tree in 195\$ (Table 3). This observation supports the statement common among pecan growers that if you "beat down" the scab in one year you have a good start on scap control for the next year. A grower planting a new orchard may well keep this in mind also and avoid planting scabby trees or dip scabby trees in dormant-spray strength Santobrite for a few minutes before planting. In deciding how much scale control will be economical it is useful to have some measure of the amount of damage scab causes. Studies on the variety Western have shown a relation between the percentage of the mature nut shuck that is covered with scab and the number of nuts (shuck included) per pound, (Table 4). The more scab present, the more nuts are required to weigh one pound. Although scab infections covering as much as 20% of the surface of the nut shucks do not materially increase the number of nuts per pound (Table 4) the effect of such a scab level the next season is also an important consideration, (Table 3). Therefore, a sufficiently extensive scab control schedule should be used each year so that the scab level is always kept as low as possible. At present in Oklahoma our recommended complete spray schedule for scab control consists of 1 or 2 dormant applications of Puratized Agricultural Spray (5 pt./100) or Santobrite (4.5 lb./100 plus 1% dormant oil emulsion) followed by 4-5 summer applications of Zerlate (2 lb./100). Because Puratized Agricultural Spray and Santobrite are very poisonous, we recommend that livestock not be grazed or forage used in orchards receiving sprays of these materials for the rest of the growing reason. None of the contact fungicides listed in Table 1 have caused damage to the pecan trees, and nuts from trees sprayed with Puratized and Corona have been found to be free of mercury and arsenite, respectively. However, the green forage plants beneath the trees are killed or stunted by the spray matrials, but re-establish themselves within a few months. A minimum spray program in Oklahoma in ungraced orchards might well substitute 1 or 2 dormant sprays for the pre-pollination applications and then use 3 or 4 summer applications beginning with the case-bearer sprays. In dry years and in an orchard with a low scab carryover even fewer applications might do the job. In wet years, with plenty of sources of scab left from the previous year, a full program of 2 dormant plus 4 summer sprays might well be required. It seems likely in any event that dormant spray of a contact fungicide can satisfactorily replace repollination applications. As safety precautions are established for relatively inexpensive materials like Corona CM-220 and Santobrite, particularly the latter which seems very promising, and as new materials which do not have a residue toxicity to livestock are found, the use of dormant sprays in the winter can be added as a part of the regular spray program for scab control in commercial orchards which are grazed. Table 1. The influence of dormant sprays on pecan scab on the variety Squirred in Paden, Oklahoma, 1953 and 1955. Scab level on Scab level on nuts at end Dormant spray Concentration Zerlate summer holdover lesions of season material and No. per 100 gallons applications (Zerlate oniy (Zerlate only of applications() of water also used? =100) =100)________ ____________________________1955_____________________________________________ Corona CM-220 (1) 3-lb. Yes 3 45______ ____________________________1955_____________________________________________ Corona CM-220(2) ground spray¹ 3 lb. Yes 3 0.4 Dowcide G² (2) 4.5 lb. Yes 0 12 Corona CM-220 (2) 3 lb. Yes 3 24 Puratized Agricultural Spray (2) 5 pt. Yes 0 36 None (check) --- Yes 100 100³ Corona CM-200 (2) 3 lb. No 3 236_____ ------------- ¹The entire ground area in a block of 5 trees was sprayed, as well as the trees themselves. ²Dowcide G contains the same active ingredient as Mnosanto Santobrite. 1.0% by volume of a dormant oil emulsion was added to the final spray mixture in either case. ³Nuts from trees sprayed only with Zerlate averaged 2.5 scab lesions/nut. Table 2. The effect of omitting sprays from the Zerlate scab-control schedule, variety Western, Paden, Oklahoma, 1953 Scab level on nuts at Date of omitted the end of the season Spray (unsprayed level=100) ----------------------------------------------------------------------------------------------------------------- April 25 7 June 8 23 June 26 33 July 23 38 August 21 32 None omitted 10 ----------------------------------------------------------------------------------------------------------------- Table 3. Scab levels on nut shucks in 1955 and 1956 from Squirrel variety pecans receiving various fungicidal sprays in 1955, Paden, Okla. Average number of scab lessions per nut shuck1 on ------------------------------------------------------------------------------------------------------- Spray treatments in 1955 8-18-55 9-27-56 Corona CM-100² + Zerlate 0.01 0.0 Puratized Agricultural Spray + Zerlate 0.9 0.1 Dowcide G + Zerlate 0.3 0.6 Corona CM-200 + Zerlate 0.6 1.3 Zerlate 2.5 2.9 Corona CM-220 5.9 4.6 No fungicides 88.7 14.6 Least significant difference: 5% level 3.7 10.0 ------------------------------------------------------------------------------------------------------- ¹Coefficient of correlation(r) = + 0.838** ²The ground between these trees throughout this entire block of trees was sprayed with this material. Table 4. Influence of pecan scab on nut weight, variety Western, 1953. Percentage of shuck surface Number of nuts(shuck scabbed in September + nut) per pound¹_________ 0 35 20 40 40 47 60 58 80 74 100 104 ------------------------------------------------------------------------------------------------------- ¹Figures were obtained by means of a regression equation.
