Copper Use, Fungicide Resistance Management and Other Orchard Plant Disease Issues as a New Season Begins

Posted: March 21, 2011

Mid March is the perfect time to start taking certain actions that can have major effects in the apple scab, fire blight and bacterial spot disease situation in your orchard this year.
Dr. Henry K. Ngugi, Plant Pathologist, Penn State Fruit Research and Extension Center

Here, I discuss my current thinking about the prevailing situation in most Pennsylvania apple and peach orchards with regard to these important diseases and suggest some actions that I think will affect the effectiveness of the actions that many of you will likely take as part of your disease management programs.

For apple scab, fire blight and bacterial spot and many other plant diseases, there is an underlying principle to remember; within the season, disease development is heavily dependent on the amount of initial inoculum available to start an epidemic. Actions taken early in the spring should therefore aim to destroy as much of that initial inoculum as possible. Specifically for apple scab, fire blight and bacterial spot, initial inoculum mostly comes from last year’s disease. We must therefore assess the level of disease last year to get a feel of how much initial inoculum we will be dealing with this year. Most orchards had low to moderate apple scab pressure in 2010 so it is fair to assume that you are starting the season with low to moderate overwintering inoculum levels. Likewise, fire blight and bacterial spot pressure were low to moderate, although more variation among orchards and, even within orchards, occurs with these diseases so this generalization should be treated with caution. It is also the case that the apple scab populations in most of our orchards have moderate levels of resistance to older DMI fungicide products, and as I emphasized in our just concluded winter meetings, the shift towards increased resistance to these fungicides is a continuous process. With this background, what actions should we be taking in the next few weeks?

Copper Applications in Apple and Pear Orchards

Traditionally, we have recommended the use of “dormant copper” applications as a tool to manage the risk of fire blight. Although empirical data are hard to find, we continue to believe that the use of dormant copper sprays until ½ -inch green is an effective way to kill fire blight bacteria as they multiply and begin to emerge from cankers from infections that occurred the previous year. Orchard blocks that had fire blight last year should thus be high on the priority for dormant copper sprays. Another good reason for recommending dormant copper sprays in apple orchards was revealed in a study just completed by my graduate student Emily Pfeufer. In a survey of commercial apple orchards to determine levels of resistance to DMI fungicides, we also polled growers on whether they were using dormant copper sprays or not. To our pleasant surprise, on average the incidence of isolates resistant to DMI fungicides was significantly lower in orchards where the grower reported using dormant copper (Fig 1). Whereas the specific reasons for this association between the use of dormant copper in orchards and reduced incidence of resistance to DMI fungicides remains unclear, my speculation is that it is due to two reasons: (i) copper destroys overwintered inocolum, and (ii) the inoculum destroyed at this time is the most important in terms of resistance development. The primary scab inoculum targeted by dormant coppers is primarily composed of populations that survived last year’s spray program probably because it carried genes for resistance to the fungicides used in 2010. Additionally, the primary scab inoculum is also mostly composed of sexual spores (ascospores) which are likely to have acquired resistance genes through mating with scab strains carrying resistance genes.  Regardless of the biology in play here, my suggestion to you is that this is good enough evidence for those who were not planning on dormant copper applications to reassess their position.

Copper Applications in Stone Fruit Orchards

I continue to recommend the use of dormant copper in stone fruit orchards as a strategy for bacterial spot control. Aside from helping reduce bacterial spot incoculum, dormant copper is also recommended for leaf curl control so in effect you manage two diseases with the same application. The most common question I encounter regarding dormant copper applications in peach and nectarine orchards is about the most effective timing between fall and early spring. We have attempted to address this question in field experiments but data have been variable (Table 1). In the absence of better data, I continue to think that early spring applications (i.e., those made in the next few weeks) will provide the best value for your money.

For those with sweet cherry, the effectiveness of copper on bacterial canker has been highly variable at best even though for now it is about the only product available for this disease. For both pome and stone fruits, another question that I regularly get relates to the choice of copper products to use. My view is that for dormant applications, all copper products are about the same. Other experts may dispute this but I have not seen data to suggest otherwise. The key to getting the best results is two-fold: (i) delay the application as much as safely realistic, and (ii) ensure the best coverage possible.
That is, if you can spray the pomes at silver or green tip, you are better off waiting until green tip. Copper is a contact biocide; the better the coverage the better the results…it is that simple. In any event, it is critical that label instructions for the particular copper product you elect to use be strictly followed.

Managing Apple Scab in Orchards with Moderate Resistance to DMI Fungicides

All other things being equal, DMI fungicides are the best weapon we have against powdery mildew of apple. Therefore, your primary apple scab program will be influenced, to some extent, by the level of powdery mildew you anticipate. Those who attended my winter meeting talks will no doubt recall that even in orchards with moderate levels of resistance to DMI fungicides, we recommend the use of specific DMI products where serious problems with powdery mildew are anticipated. For the DMI fungicides currently labeled for apple, it appears that products that are most effective for apple scab control are not necessarily the most effective on powdery mildew (Dr. Dave Rosenberger, Cornell University’s Hudson Valley lab, personal communication). If you have major concerns with powdery mildew therefore, you will want to consider using the DMI products considered excellent for mildew in the early sprays between the pink and petal fall stages before rotating them out with other chemistries, e.g. the strobilurins. Used at full label rate with a protectant fungicide, these DMI products should provide good levels of scab control given the level of resistance detected in most commercial orchards in our recent survey. Growers will have to assess how bad their resistance situation is for each of their blocks.
Table 1. Effect of dormant copper application on bacterial spot severity during cover sprays.
                                                                  Disease severity (%)
Treatment                                             Site A                          Site B
Fall copper                                        8.1 ± 0.338 A              10.0 ± 0.391 A
Spring copper                                    7.9 ± 0.335 A              10.9 ± 0.430 A
No copper                                         8.4 ± 0.609 A              12.1 ± 0.831 B
Values are means and standard errors of four replicate trees. Means within a column followed by same letter not significantly different (P =0.05).

While we are discussing resistance, I cannot overemphasize that our goal should be to prolong the utility of the site-specific products (DMIs, Stroby’s, etc,) to the extent possible because new products are not becoming available fast enough to keep pace with the development of resistance to the existing ones. The recommendations we regularly have provided elsewhere (e.g., in the PA Tree Fruit Production Guide) on management of resistance such as use of proper rates, ensuring proper coverage and spray intervals, mixing with protectant fungicides, rotating with nonrelated chemistries, etc, etc. remain very useful. You are the best steward of your orchard and your actions will determine the long-term sustainability of your disease management programs. Finally, the effectiveness of your fire blight program will depend on proper timing of your streptomycin applications during bloom. Hopefully most of you have access to a disease forecasting service such as provided by SkyBit or commercially sold with weather equipment as these provide the easiest way to appropriately time streptomycin applications. We shall try to post warnings on the web but our data are only truly applicable to orchards around Biglerville. Have a great and disease-free season!