(A) Urea/Lime for Reducing Apple Scab and Apple Blotch Disease Overwintering Inoculum; (B). Delayed Dormant Copper for Apple, Pear & Peach

(A) Urea/Lime for Reducing Apple Scab and Apple Blotch Disease Overwintering Inoculum; (B). Delayed Dormant Copper for Apple, Pear & Peach

I. Apple Scab and Marssonina Leaf Blotch – why we had issues in 2022 and how to reduce overwintering inoculum? With the frequency and the amount of rains we received in 2022, managing scab and Marssonina Leaf Blotch (MLB) aka Apple Blotch disease was truly challenging. In the Figure 1 below you can see a large number of curved dark blue lines at the bottom of the the big central graph indicating to you a very high frequency or rains in 2022. On top of that, the variable heights of these curved lines indicate the amounts of rain per each rain event (the rain amount is read off of the blue numbers scale on the far right of the central graph: “Inch rain”). What is striking is that we received three events with 2-inches or more rain on May 5th into 6th, 22nd June, and 10th Aug. In such conditions during the year, alternate row middle spraying concept can fail to provide adequate fungicide coverage unless the spray interval is reduced. This would allow apple scab and MLB disease to infect and cause problems until the rest of the season. Folks who had issues with apple scab, MLB, apple bitter rot and/or Glomerella leaf spot in 2022, they should consider implementing complete spray applications every second or third cover during the summer. That could improve management efficacy of materials applied.

Figure 1. Summary of historical weather conditions in 2022 for Winchester, VA, collected from an on-site weather station. The upper large graph: red line represents temperatures (left y-axis in red), curved blue line/s represent rain events indicating amount in inches (right y-axis in blue), and gray background peaks represent relative air humidity (RH) in % (far left y-axis in black). The small bottom graph with dates (x-axis) indicates the length of periods of rain (dark blue bars) and of wetting period afterwards (light blue bars). Source: RIMpro B.V. Cloud Service, France, an interactive Decision Support System for pest and disease prediction and management in fruit and grape production.

As the fungi that cause apple scab and MLB overwinter in the dead leaf litter on the orchard floor, you can increase the efficacy of your spring fungicides by using practices and materials that allow faster degradation of the leaf litter that allows these pathogens to overwinter. Reduction of apple scab ascospore inoculum dose in leaf litter with the following methods can range from 50-70%. If there is no snow cover and mud is not preventing you to drive the the tractor, use flail mower to shred leaves the to smaller pieces – that will speed up the degradation of dead leaves by microorganisms and worms on and in the soil. In smaller orchards, you can rake the leaves from under the trees into row middles and remove leaf piles with flail mower’s mode for scalping the sod. The most used alternative is to spray apple leaves with urea (you can combine both: apply urea first and then use flail mower after several days). If you missed to do this last fall, just before the major leaf drop, you can spray leaf litter on the ground and best timing to do this is NOW: late winter but before bud break. Use rate of 40 lbs of urea / A in 100 gals of water. Turn your airblast air deflectors downward and/or turn off the top nozzles. This will allow spray mist plume to lift the loose leaves on the orchard floor and coat them with urea. Once done, wash and rinse well your spray equipment since urea can wear up any rubber parts, washers, and gaskets in the sprayer, especially the sprayer pump diaphragm. If you are an organic grower, urea is not a choice you can use. Apply dolomitic lime at a rate of 2.5 tons /A, which can also be used in conventional orchards. Lime is best used after leaf drop in fall or early in the winter, but you still have time. Lime increases the pH or basicity (opposite to acidity) of soil surface promoting microbial activity. This helps apple leaf litter breakdown. You can use powdered lime spreaders as the most efficacious way of applying dolomitic lime.


II. Delayed Dormant Copper in Apple. Copper in early bud break is the key component of all good fire blight management programs, even though it also helps with apple scab sanitation. Copper ions released by rain from different copper materials kill fire blight bacteria on the bark and unfolding green plant surfaces. However this is only occurring if fire blight bacteria emerge from cankers (Fig. 1) during the time when copper deposit is still present on the bark (and not washed off by rain). Copper products differ widely in the availability of free copper ions released on wet plant surfaces. Copper sulfate crystals (Blue Stone) is highly soluble in water and quickly releases copper ions thus having the highest potential to cause phytotoxicity if you are beyond 1/4 inch green in flower bud development. Due to this issue, highly soluble copper active ingredients are mixed with lime or gypsum to bind copper ions and slow their release over time. The fixed copper materials like copper hydroxide, are less soluble in water and slowly release copper ions with each wetting event. Copper is sprayed as a water suspension, where active ingredient particles persist on plant surfaces after the spray solution dries. With each rain, dew or spray event, active copper ions are gradually released from copper deposits providing residual protection against fire blight and scab. However, if 3 inches of rain or more accumulate in single or several close-by rain events, almost all the copper residues are washed off the bark and this will not provide the fire blight protection from primary inoculum emerging from fire blight cankers. Sometimes, if heavy rains are predicted (1.5 to 2 inches), experienced growers might use high label rate of copper for this time of the year so that at least some residue stays on the tree bark through the rain and lasts beyond 1/4 inch green.

Figure 2. Fire blight canker on trunk of cultivar ‘Idared’. Cankers serves as fire blight infections source in spring the following year (Photo by S. G. Aćimović 2008).

Delayed dormant copper is applied late in tree dormancy, i.e. from bud break up to the 1/4- to 1/2-inch green bud stage (if the label permits the latter two). Most labels do not allow use at 1/4 or 1/2-inch green bud stage due to possibility of copper to cause bud injury. Keep in mind that it is risky to apply copper at 1/2-inch green, as you easily might miss that window while you are thinking that you are at that stage. Missing this window, if the label allows it, will cause phytotoxicity and injure the floral buds. The best practice is to apply copper earlier at or from Silver Tip, Green Tip, up to 1/4 inch green (for e.g., Green Tip is when 50% of floral buds are at green tip). To determine at which stage are you, count randomly selected 100 flower buds on at least 15 different trees (repeat this over several days) and tally up the buds into categories: Green Tip buds, Silver Tip buds, 1/4 inch green buds. Once you sum each category totals, determine the percent of each catrgory in relation to the 100 total buds. If you are at 50% of buds at green tip that is the Green Tip stage. The same applies to silver tip or 1/4-inchvgreen stage. If you are beyond 50% of buds in your desired stage, you are likely no longer at that stage. Some softer forms of copper like Cueva, could be an option if you miss the optimal stage, but always follow the label in applying copper.

Delayed dormant copper is applied to (A) reduce fire blight bacteria on the bark and unfolding green tissue surfaces of buds disseminated from carryover fire blight cankers; (B) Deposit enough copper residue to hopefully stay on the tree surfaces until the fire blight bacteria emerge from cankers with warm weather (this can start as late at petal fall), and (C) Avoid toxic effects of copper i.e. russetting on expanding green tissue, including the base of the flower buds that will eventually develop into fruit. Depending on the spring weather conditions, achieving one or more of these goals can fail, thus reducing the copper effectiveness or risking bud injury if applied too late or at high rates. Excessive spring rains can wash off all the copper residues if 3 inches of rain or more occur between spray application and pink bud apple growth stage. That is why folks must apply streptomycin during bloom when fire blight models predict infection (you cannot rely only on copper). If only low concentration of copper ions stay on bark when bacteria emerge from fire blight cankers, and if these residues are additionally diluted by bud development from green tip and bloom, they might not be sufficient to kill the bacteria. Next, in most years, fire blight pathogen emerges on canker surface at the end of bloom and start of shoot growth, when it warms up and sap flow speeds up with green tissue growth. By that time, little or no copper deposits remain on the bark to control the bacteria. The purpose of delayed dormant copper spray/s is to provide an inhibitory barrier of copper ions over bark and bud surfaces to prevent fire blight pathogen E. amylovora from colonizing these areas. If you did not have fire blight symptoms in your orchard in the last 4 years, copper spray is not as necessary. However, if you did have ANY fire blight in recent past (2-3 years), a copper spray applied from silver tip to 1/4-inch green is a must and will also provide some protection from apple scab. Copper at this time is equal in effect against apple scab as one application of mancozeb is. One copper spray for scab will last about 7-10 days, so do not rely on it later than that. Delayed dormant copper will not eliminate the need for applying streptomycin at bloom when fire blight prediction models report infection is imminent.

Oil for insect and mite control and copper are compatible for mixing, unless a specific copper product label warns not to mix it with oil due to incompatibility. The standard for control of overwintering eggs on apples is 2 gal/100 gal (2%) at green tip until half-inch green stage, or 1 gal/100 gal (1%) at tight cluster (use 1.5 gal/100 gal if the buds linger between half-inch green and full tight cluster). In the past, when oil was cheaper as it was not that refined, growers would use 3 gal/100 gal (3%). If you had a high insect pressure last year and want to make sure you are protected, you can apply two oil sprays. However, if you have been satisfied with the insect control in 2022, applying oil at just 1 quart/100 gal mixed with copper to act as a spreader and improve coverage will be cheap and sufficient. Then, at at half-inch green apply your second oil at 2% or 1% at tight cluster. Do not apply copper in acidic solutions. Do not mix copper with products that lower the spray solution pH. Acid solution leads to more immediate release of copper ions and higher potential for copper toxicity to green tissues. If you can, apply copper in high-gallon sprays to get good coverage of the entire tree for maximum efficacy. If you get delayed from applying copper and first major scab infection period is predicted soon, to start both the fire blight and scab program you can can mix copper, 1 quart of oil, and an anilinopyrimidine (AP) fungicide (e.g. Scala, Vangard) and apply this mix within 72 hr from the time when rain began triggering first scab infection period. However, the copper in this mix would be justified if fire blight is a concern due to recent history.


III. Delayed dormant copper in pears. Use copper ONLY when trees are still dormant late in winter and up to bud burst (no later). Applying copper beyond bud burst can cause leaf burning especially if slow drying conditions occur. Add oil at 1 quart per 100 gal of actual spray solution in the tank and do not concentrate the oil. If using Bordeaux mix, add the oil after adding lime, but before topping up to final volume. One quart of oil can increase the efficiency of copper compounds but will not control psylla. Therefore, for psylla apply a separate oil application, or mix in copper with 3 gal oil/100 gal water.

CAUTION: If frosts are predicted to occur or have occurred after green tissue emerges from buds, be extremely cautions in deciding on applying oil mixed with copper near frost events. This is necessary if frost(s) occurred when buds are near the 1/2  green stage i.e. when significant amount of green tissue is exposed. In these cases, avoid using copper in mix with oil before or after announced frost conditions. Both oil or copper alone, but even more in a mix, can cause leaf and fruit cluster injury due to high rate of uptake into the green tissue. Uptake of these products into the green tissue occurs through wounds opening when ice melts. The mix is more damaging because oil acts as a penetrant and improves further uptake of copper in an already frost injured tissue. The best option is to leave a several days break before or after the frost event/s.

NOTE: At this time of the year, combine removal of fire blight cankers with pruning as that will reduce primary bacterial inoculum in the orchard. Cankers are the main sources for infection of flowers. Fire blight cankers formed on large branches, limbs and trunks have a higher chance to emerge viable fire blight bacteria for pear and apple flower infections. Therefore, removal of fire blight cankers during dormant pruning is the key practice that will reduce the chance for serious fire blight outbreaks in the future growing seasons. Regardless of how meticulous the pruning crews are in removing cankers during the winter pruning, some cankers always remain unnoticed on old branches or on the trunk. Carried over into the next season, cankers pose a constant threat by providing inoculum for new fire blight infections during bloom or shoot growth. It has been determined that only 1 to 4 cankers per 2.5 acres are enough to allow fire blight infection renewal in the spring. A period of pome fruit tree susceptibility lasts from when the very few first flowers open on earliest apple or pear cultivars/s on you farm, all the way up until the terminal buds set on shoots on these trees.


IV. Dormant copper and ziram in peach. Copper for leaf curl should be applied while trees are dormant, late in winter, up to and during bud swell (bud break). Copper aims to reduce the leaf curl fungus spores that overwinter on the tree. Infections take place in the spring as the buds open. This fungal pathogen infects buds during rain events from bud swell to bud opening. Very long cool wet periods during bud burst slow bud development and thus lead to severe peach curl infections. Copper products also give some suppression of bacterial spot. Use at least 4 to 8 lb of metallic copper per acre. You can combine Ziram and copper, but do not apply this mix during bud swell (bud break). If you missed the window for using copper and you suspect based on cool and wet weather that infection has already occurred, Bravo or Ziram are better than copper as they have efficacy after infection. Ziram for leaf curl should be used during dormancy, after leaf drop in fall, and prior to bud swell.