Field management of postharvest rots reviewed

A holistic management approach to
postharvest disease prevention is advocated.

By Dr. Bill McPhee

Many excellent researchers, including those in Washington, Oregon, and British Columbia, have studied all areas of postharvest pathology, including contamination of dump tanks and flumes, field inoculum levels, chemical controls such as drenches, biological controls, etc., on all varieties of apples and stone fruits.

Despite the research, there is much concern in British Columbia about the high incidence of postharvest rots costing growers millions of dollars, and, I assume, there are concerns in other areas also. Currently, the major postharvest problem in British Columbia is related to a Botrytis sp., but, in my experience, the cause of problems has varied from year to year. We have had times when bull's-eye rot was prevalent and times when Corticium rot, or Botrytis or Alternaria, etc., were causing concern. The question that comes to my mind is, why are we having the trouble we are having today in spite of our best research efforts?

I do not think the blame is on the researchers, their programs, or research funding. Rather, the problem relates
to the extension programs and the lack of application of the research and extension data available.

Some of the research that is being carried out today is very exciting from an extension point of view. Population studies are beginning to unveil the interaction between pathogens and nonpathogens in the field. These studies, in combination with studies on biological controls, are opening a whole new area of what I refer to as "natural control," natural control being the management and utilization of natural populations within the orchard system.

I began to research this holistic approach to postharvest disease problems in the early 1980s. I understand now, after 12 years of extension experience, that this early work, carried out at the Summerland Research Station, was naive. The experience of working in extension has broadened my horizons to the extent that I now feel more capable of applying a holistic system to the practice of postharvest disease prevention. Application of the holistic philosophy involves a management approach which pulls together all the orchard information relevant to the stability of the individual fruit.

Integrated systems management

Integrated systems management (ISM) is the system we are attempting to implement as a tool in the prevention of postharvest problems. The ISM system is a coordination of information gathered on our apple blocks and the fruit from each block, including tree condition, fruitlet analysis, early detection of disease potential, etc. Together, these factors give a good indication of the rot resistance inherent in the fruit, and all contribute to the ISM system. The single most important factor in the ISM system is effective orchard monitoring!

Once all the data about each orchard is loaded into a computer, the next step is to relate it to an electronic mapping system, a global positioning system (GPS). The same mapping system will be connected to other relevant information about the specific area. In this way, we have all data spatially coordinated. Once this is done, then, if there are distribution patterns or other correlations, they can be expressed and used in control strategies where applicable.

Coordination between research and extension is a key part of this management system. It is customary in the United States to have cross appointments in extension and research, but this is very unusual in Canada. There is a distinct separation between research, which is a federal responsibility, and extension, which is a provincial responsibility. The advantage to having these two factions working as a team is obvious. The ISM system is weighted heavily toward extension but is dependent on research advances for improved sophistication.

Types of field problems

There is no question that some of our storage disease problems are truly generated postharvest, but many are related to field conditions, environment, orchard management, cultural practices, etc. In other words, they are diverse, and often, the true cause of the problem is not obvious. Let's look at four examples of problems that appear simple, and then let's examine these in some detail to determine the true cause.

Problem 1 was a major rot problem in one of the storages. The apples had gone through the sorting system and the culls were taken out, then returned to bins for storage prior to juicing. After a few days, it was noted that the apples in these bins were covered with Penicillium. The problem did not show up in fruit from the same lot that had been packed and shipped, but only in apples in the bins returned to storage. The packing house personnel inquired about the problem and requested information on control of the contamination that, to them, was obviously related to contamination in the storage room.

Problem 2 was with a lot of Golden Delicious apples that came out of CA storage in March. The particular lot of fruit was chosen to fill a large order based on the grower's history of delivering large, clean fruit. As packing began, it was clear that this fruit was heavily infected with bull's-eye rot (Gloeosporium perrenans/Pezicula malicorticis). The fruit was pulled, and the problem turned over to the field service. It appeared to be an obvious case for a field treatment to control bull's-eye canker.

Problem 3 was the sudden increase in the incidence of Botrytis rot in the Okanagan Valley, particularly in Red Delicious lots, over the past two years. This has become a major concern to the packing houses, but why?

Problem 4 outlines another example of a rot problem with a unique twist, the incidence of moldy core (caused mainly by Alternaria spp.). This was never a major problem, because moldy core infections are not aggressive and seldom move out of the core area and into the flesh. You could eat an apple infected with moldy core and, by throwing away the core, never realize that the core was moldy.

The ISM approach

Let's now go back and look at these four examples. Why did they occur? Could they have been avoided? How would ISM apply?

Problem 1. At first glance we have a storage room contamination. However, a closer look indicates that there are more subtle factors involved, and observation of the sorting line gave the answer. The apples culled from the line were dropped down a chute where they hit a foam rubber backing and gently dropped onto a rubber belt. The belt then transported the apples back to the flume system where they were floated to a bin filler and returned to storage in bins.

As the apples dropped down the chute, it was clear from the sound of them hitting the bottom that the foam rubber backing strategy was not working. In fact, the foam backing had worn off and the apples were hitting a sheet metal wall that had supported the foam when it was in place. The nails that had held the foam in place and the sharp metal edges were puncturing nearly every apple. The apples were being returned to a flume heavily contaminated with Penicillium. This was, in reality, an excellent inoculation system.

The solution was easy: equipment maintenance. In our current ISM system, the contamination (Penicillium) would automatically go through a screening process and be identified. Flume waters are routinely screened for levels of inoculum and regular cleansing of the flume systems occur. This approach is preferred over flume treatments such as the addition of chlorine.

Problem 2. This is a postharvest disease problem that is undoubtedly directly attributable to orchard management. It involved a grower's Golden Delicious lot chosen for packing a special order in February. The grower had a history of supplying clean, large fruit. However, in this particular case, as the bins of apples were dumped into the flume system, it was clear that there was a serious problem with storage rot. The rot was determined to be bull's-eye caused by Gleosporium perrenans (Pezicula malicorticis); but why?

This grower had one major block of Golden Delicious, and that block was known to contain perennial cankers in the trees. The cankers had been there for years, and, in fact, was a convenient spot to take students to show them cankers in the field. In spite of the history of cankers, there had never been a problem with fruit from that block in my ten years of experience with the block.

There was a general recommendation from the packing house to spray calcium chloride during the season, a minimum of three times at a rate of 200 gallons per acre of six percent strength. This grower had faithfully followed this program for seven years, but two years prior to this problem, he stopped these sprays, thinking they were unnecessary. After this problem was traced back to his block with the cankers, the grower resumed his calcium sprays, and the problem stopped.

Calcium is extremely important to the structural integrity of the cell wall, and there is plenty of evidence for its ability to decrease fungal rots in apples. Other nutrient levels in the fruit impact the storability of the fruit and the incidence of rots within the fruit. The ISM system at the packing house includes fruitlet analysis data on many of our varieties. The potential for a lot of fruit to generate a severe bull's-eye problem would be evident from the nutrient data carried out before harvest. Weak fruit (low calcium fruit) would be moved early to minimize the potential for the disease to develop.

Problem 3. The Botrytis problem is an interesting one, and is being researched at the research station in Summerland by Dr. Peter Sholberg. From a practical point of view, this is an intriguing problem because it appears to be intermittent over the years. In the early 1980s, this problem was showing up late in the storage period in Spartans and Red Delicious as a Botrytis calyx end rot. The problem was severe for two years, then declined.

At that time, there seemed to be a specific complex at work. There was always moldy core associated with the Botrytis rot at the calyx end, and, since moldy core occurs very early in the development of the fruit, it was speculated that moldy core predisposed the fruit to the Botrytis infection. In only one incidence did we find the Botrytis without the moldy core. In that case, seeds were plated out on agar to determine if the Alternaria infection was indeed there but not being expressed. The finding was that, in this case, the seed cavity was contaminated with a bacterium antagonistic to the Alternaria. Here was an example of a natural control.

In the textbooks, this is simply a case of early control of inoculum at the field level. But where is the inoculum coming from and why some years and not others? At the time, I had suspicions that the Botrytis outbreaks were more prevalent in orchards close to vineyards. This hypothesis could easily be checked using a GPS system.

In extension in the south Okanagan (1984-1994), I continued to deal with postharvest rot problems at the local level, but Botrytis rot was never a serious issue. However, since then, the incidence of Botrytis seemed to explode again. Why? Is it related to the renewed expansion of vineyards in the area?

There has been, and is, ample research being carried out on Botrytis and Botrytis control, but there are very few practical programs for the field people to follow regarding prevention of this problem. One piece of information that intrigues me is the fact that in the mid-1980s, the problem began to disappear at the time that a program was instituted by the government to remove grapes from the valley. A significant portion of the valley's acreage was removed. However, recently, as the wine industry, and subsequently, the grape plantings began to expand, the incidence of Botrytis on apples has been increasing. The normal factors that increase disease pressure, temperatures, moisture, etc., still are important, but the significance of vineyards as a source of increased inoculum cannot be ignored. The question is: How do we manage such a problem?

Problem 4. The moldy core example is a major concern to the B.C. industry because of the method of serving apples in the Oriental market. Sectoring the apple for serving exposes the seed cavity where the Alternaria can be easily detected. In routine evaluation of apple fruitlets in the spring, we record the incidence of moldy core.

Problem blocks are designated for early sales and are kept out of the Oriental markets. This approach has been an effective marketing strategy without using fungicide control treatments. However, the need for fungicide treatments in some cases is not ruled out, and studies are ongoing to test the control spectrum for newly registered fungicides. Fungicides that have the capability to control Alternaria but are also effective against other diseases such as mildew or scab, would be preferred for some blocks of fruit.

Conclusion

Proactive management of postharvest disorders, including rots is the most effective approach to control. Proactive management, however, requires an active field monitoring program that defines the orchard conditions that impact the internal quality of the fruit and the potential for storage disorders.

The whole point of this essay is to illustrate that postharvest rot problems do not fit into neat, definable categories. Research, out of necessity, controls experimental parameters in order to study one variable. In the field, there are few controls and almost infinite variability. The challenge at the field level is to study the problem within that highly variable environment.

The monitoring of field conditions and fruit conditions is the most effective way of using the information, and it can be enhanced if the information is spatially defined. In other words, if that information is analyzed within the parameters set by a GIS or GPS system, then fruit storage quality and postharvest problems can be patterned within the test area. The spatial patterns that may be elucidated by this monitoring system can tie together any field data stored in the database. For example, the suggestion above that there may be a relationship between the incidence of Botrytis calyx end rot and proximity of the problem orchard to a vineyard, would be tested very easily.

The more information that can be gathered, the more potential the prevention approach is likely to have. I am proposing an industrywide program for our industry in British Columbia. In order for this program to be sustainable, there needs to be coordination between researchers, extension specialists, field persons, and quality control personnel at the storage facilities.

Dr. Bill McPhee is northern technical
specialist for Pace International.

Copyright 1999, Good Fruit Grower,105 South 18th Street, Suite 217, Yakima, Washington 98901
Voice (509) 575-2315, (800) 487-9946, Fax (509) 453-4880

Field management of postharvest rots reviewed

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