 |
|
|
||
|
Fruit Notes |
An Easy and Reliable Procedure for Predicting Scald and DPA Requirement for New England Delicious Apples |
|||
|
Fruit Notes |
Sarah A. Weis, William J. Bramlage,
and William J. Lord |
|||
|
Fruit Notes |
The processes which control scald susceptibility and development have been, and are being, studied by many researchers. While the exact causes of scald are not known, it has been observed that a) within a cultivar, late harvested fruit scalds less than early harvested fruit, b) other things being equal, the riper the fruit at harvest, the less they will scald, c) scald susceptibility varies from year to year among fruit, even from a given tree, and d) year to year variation is (at least partly) a function of weather. Cool, sunny weather favors scald reduction, while hot weather may increase scald susceptibility. We have been studying the effects of weather on scald variability, both at the University of Massachusetts Horticultural Research Center (HRC) and at other New England locations in relation to effects recorded at various locations world-wide. Here we will report on recent findings from the New England studies. Our sites were in Ashfield, Belchertown (HRC), Shelburne, Warren, and Wilbraham, MA, Storrs, CT, Putney, VT, Durham, NH, and Monmouth ME. First, it must be recognized that many of the factors purported to influence scald are related to one another. For example, as the harvest season progresses, the fruit ripen, temperature falls, and the days become shorter. However, these factors relate somewhat differently to one another in different years, and in different locations. Our hope was that if we collected data for many years at the HRC, we could identify some of the specific factors influencing scald susceptibility, and then develop a workable general relationship of preharvest factors to scald susceptibility. The factors we have concentrated on are the following: 1) preharvest cool temperature as the number of days the apples are subjected to sub-50oF weather between August 1st and harvest; 2) a qualitative measure (sunny, partly cloudy, cloudy) of light to which fruit are subjected during the week prior to harvest; 3) fruit maturity at harvest as measured by a starch/iodine test, and 4) harvest date. Because we have been working primarily with Delicious, which are not harvested before late September, we did not experience enough hot (over 80oF) weather near harvest to evaluate that factor, even though we know it could be important. We also have not included a light factor in the reported equations because light measures were only available for the HRC. Development of Prediction Equations Between 1988 and 1993, we harvested 213 one-bushel samples of Delicious from HRC, stored them 20-25 weeks in 32oF air, and then removed the fruit to room temperature for a week before rating them for scald. Scald rating consisted of examining each apple in a box and recording the percent of apples developing scald in that box. A sample was considered very scald-susceptible if more than 60% of fruit in the box developed scald, and was considered scald-resistant if fewer than 20% of the fruit developed scald. It should be noted that any sign of scald was recorded, and some fruit in this latter category would not have been downgraded. The following equations, reported in Fruit Notes 61(4) were generated: Equation 1: If [8.36 - 0.320(harvest date as number of days after 8/31) + 0.0546(number of preharvest days < 50oF) - 0.0550(harvest starch score)] > 0, then fruit are very scald-susceptible. Equation 2: If [-11.8 + 0.414(harvest date as number of days after 8/31) - 0.0298(number of preharvest days < 50oF) - 0.708(harvest starch score)] > 0, then fruit are very scald-resistant. If a sample fits neither of these categories, it is considered to be of intermediate scald susceptibility. Please note: There was a typographic error in one equation in Fruit Notes 61(4). Those that we present here are the correct equations. The numbers you get from these calculations are called Indices. These Indices were tested in 1995, 1996, and 1997 a) for their ability to accurately place samples in the correct scald category and b) to see if they could be used as guides for application of the scald inhibitor, diphenylamine (DPA). We also are interested in relating the scald potential of controlled atmosphere (CA)-stored fruit to the scald Indices. In 1996 and 1997, we stored fruit from the HRC in CA, as well as in air storage. Scald symptoms on the CA fruit were roughly parallel to symptoms on air stored fruit, but affected a significantly higher percent of the fruit. It is possible that this is because CA fruit were stored for 28 to 29 weeks, while air-stored fruit were kept for 25 weeks. In CA, the specific atmosphere should influence scald potential. Our storage atmosphere was 2.8% O2, <2% CO2. Lower oxygen concentrations ought to reduce scald, but we do not yet have adequate data to address specific questions regarding use of the scald Indices to determine DPA requirements for CA-stored fruit. Ability of Equations to Predict Scald Figure 1 shows how well the 1995, 1996, and 1997 samples fit the equations developed using the pre-1995 data. These samples included fruit from Storrs, CT, Putney, VT, Durham, NH, and Monmouth, ME, as well as the MA locations shown in Figure 2. In each category, observed scald was only slightly different from predicted scald, indicating successful application of the equations to estimate scald potential at the time of fruit harvest. Very Scald-susceptible Fruit Figure 2 shows the most scald susceptible samples from Massachusetts,which were those from the first harvest at each location. In all locations and years scald did develop on more than 60% of these fruit. However, responses to DPA varied by location. The highly scald susceptible fruit from the Warren MA site responded to 500 or 1000 ppm DPA to a much greater degree than did the corresponding fruit from other sites. (The only statistical comparison which could be made was with HRC and there the difference was significant at odds of 1:100). Yet overall, 2000 ppm DPA would have to be recommended for confidence that scald would be controlled in these highly scald susceptible fruit. Note that all the fruit samples represented in Figure 2 were harvested before 24 September, and Delicious are not normally harvested that early in New England. Results for samples harvested during the commercial harvest period will be represented below in Figure 3 and Tables 1 and 2. Intermediately Scald-susceptible Fruit When we moved on to consideration of the next category of fruit, those of intermediate scald-susceptibility, we began looking closely at scald severity, as well as whether any scald-like symptoms were evident. Most of the samples of “very scald-susceptible” and “intermediately scald-susceptible” included fruit displaying scald of varying degrees of severity. Scald would clearly render some of the fruit unsaleable in the fresh market. Other fruit, however, displayed scald, or scald-like symptoms, which would not be noticeable to the casual observer, and would not likely downgrade the fruit. The “very scald-susceptible” fruit tended to have enough severe scald that there was no question but that 2000 ppm DPA was needed if the fruit were to be stored for 25 weeks. In the fruit of intermediate scald susceptibility (or less), there often were few fruit with severe scald. In some cases we had difficulty determining if a given apple did or did not have scald. Therefore, we used a rating system which separated fruit showing any discoloration that might possibly be scald, from fruit with more distinct external browning which would clearly reduce the grade of the fruit. In Figure 3, the term “distinct scald” is used. This scald was not always severely disfiguring, but it always was clearly noticeable. The term, “Any Scald” is used in Figure 3 (and Figure 4) to denote fruit which showed any scald-like external browning. We did observe differences in the appearance of scald on fruit from different sites. Scald on the HRC Redspur in particular was difficult to rate, because the early harvested fruit tended to have a brownish cast which was not necessarily scald, but did not occur when the higher concentrations of DPA were used; this brownish cast did not appear on fruit from the latest harvests. In contrast, the Sturdeespur from Warren showed scald very clearly if it was present at all. The appearance of scald was also variable among the different striped strains of Delicious we harvested in Ashfield in 1996. In fruit from some trees, the scald stood out, while in fruit from others, scald blended with the fruits’ coloring, making it less noticeable. Figure 3 shows the effects of 500 ppm DPA on fruit predicted to be of intermediate scald susceptibility. These fruit all were harvested from September 30 to October 3, when commercial Delicious harvest is expected to begin. From 0 to 3% of the 500 ppm DPA-treated fruit shown in Figure 3 clearly displayed scald. It should be noted for those who find 3% scald too much, that the 1996 HRC Delicious, which show 3% scald in Figure 3, also showed 3% scald following storage when 2000 ppm DPA was applied. Overall, 500 ppm DPA provided adequate scald control; use of 1000 to 2000 ppm DPA was not any more beneficial and constituted overuse of the chemical on these intermediately susceptible fruit. Fruit With Low Scald Susceptibility The second equation presented earlier in this report identifies fruit of low scald susceptibility. This category should represent fruit with little or no need for DPA treatment. Identifying these fruit is of particular interest because 1) scald control treatment is expensive and inconvenient to apply, and 2) the materials used are not acceptable to an increasing number of markets. Clearly, if 20% of fruit in a lot develop scald, this is unacceptable. However, when we attempted to generate equations for forecasting small amounts, for example, less than 10% scald, we were unsuccessful; there was simply too much variation in scald development in the vicinity of 10%. We therefore developed the equation to predict under 20% scald because it produced consistent results. If the Index for the “scald resistant” equation (Equation 2) is greater than zero for a given day, location, and starch score, then fewer than 20% of fruit harvested on that day will be predicted to scald after 25 weeks of storage. In addition, the higher the Index, the greater the likelihood that fewer than 20% of fruit will scald. It also follows that the higher the Index, the less scald one would expect. Tables 1 and 2 show how many fruit developed any scald-like browning (Table 1) and how many developed “distinct scald” (Table 2). As the Index increased, the amount of scald did indeed decrease. There was some variability among orchards and years, but the trend was there. What this means is that an individual could base scald control measures on personal experience about how much scald had developed on fruit from this site in previous years, and on how much risk the grower was willing to accept, using the tables shown below as a guide. If the Index for Equation 2 is greater than zero, low susceptibility exists. As the number becomes increasingly greater than zero, the risk of scald becomes progressively smaller. A Simple Procedure for Using the Prediction Equations The equations presented earlier in this article can be intimidating. However, they can be rearranged to produce an easy-to-use system for tracking scald susceptibility during the harvest season. Example: Delicious were harvested on October 1, the temperature had gone below 50oF on 7 nights since August 1, and the starch score at harvest was 2.2. Using Equation 1: 8.36 - 31(0.320) + 7(0.0546) - 2.2(0.0550) = Index 1 so, Index 1 = -1.30 Since the Index is less than zero, less than 60% of the fruit are predicted to scald after storage if no DPA is applied. Using Equation 2: -11.8 + 31(0.414) - 7(0.0298) - 2.2(0.708) = Index 2 so, Index 2 = -0.73 Since this Index also is less than zero, then more than 20% of fruit are predicted to scald. Therefore, if these fruit are stored in air at 32oF for 20-25 weeks, between 20% and 60% of the fruit can be expected to scald if no DPA is applied. This scald can be controlled by application of 500 ppm DPA, as seen in Figure 3. Post-storage Scald Development One of the concerns about scald is that while it may not appear on fruit on removal from storage or when packed, it may appear later, before fruit are consumed. We had not intended to address this issue, but we would like to report here that we did not see a great deal of scald developing during the week the fruit were kept at room temperature. In 1996 and 1997, we inspected some of the fruit at the time of removal from storage, as well as after 7 days at room temperature. Figure 4 shows that on air-stored Delicious, this post-storage increase in scald was not significant. Note that sometimes more scald was discernible at removal from storage than after a week at room temperature. This is an indication of the difficulty we sometimes had in determining if fruit discoloration was or was not scald. It should be made clear that not all cultivars are like Delicious in this regard. For example, we have observed repeatedly that Cortland may show little or no sign of scald at removal from storage, but after 7 days at room temperature, much scald is present. Conclusions We conclude from this research that forecasting scald susceptibility on air-stored New England Delicious is feasible, and we believe that use of these prediction models can lead to more efficient use of DPA. Results of 3 years of tests of the models developed from 6 previous years’ data indicate that using a calendar, a min-max thermometer, and a starch test for maturity, one can use these equations to effectively predict high, intermediate, or low scald susceptibility of Delicious apples harvested at a given site on a given day anywhere in New England. Furthermore, the equations predict the need for DPA: highly susceptible fruit require 2000 ppm, intermediately susceptible fruit require only 500 ppm, and low-susceptibility fruit, particularly those with an Index greater than 1 in Equation 2, have no need for DPA treatment. We offer a system for determining which fruit need 2000 ppm DPA, which will be protected by 500 ppm, and which may be stored without DPA treatment, and with a minimum of concern for post-storage scald development on air-stored fruit. As demonstrated by the data in Tables 1 and 2, it is possible to choose an Index higher than zero in Equation 2 as a demarcation between groups of fruit which receive 500 ppm DPA and those which receive none. This can be especially useful if experience has shown that the fruit from particular trees are especially scald susceptible, and allows flexibility in determining at what point to stop applying DPA. Acknowledgments The authors would like to thank the following people for their assistance in selection and provision of fruit and preharvest information for developing and testing scald prediction models: Dana Clark, Evan Darrow, David Kollas, William G. Lord, Wayne Rice, James Schupp, Joe Sincuk, Tim Smith, and Mark and Bob Tuttle. Technical assistance by Irene Clark and Laura Lee Jones is much appreciated. We would also like to thank the Massachusetts Fruit Growers’ Association, the New England Tree Fruit Growers Research Committee, and the Washington Tree Fruit Research Commission for their financial support of this research. |
|||