Fungicide Use Considerations in Frost Impacted Orchards

May 6 is still far away into the future to trust weather forecast accuracy. However, if any rat-tail bloom still stays open on young trees, on that day a streptomycin application will be needed in Roseland, VA, and Sperryville, VA (infection is predicted as of today for 6 May in these two locations). Keep an eye on NEWA EIP model and stay vigilant. Apple scab will not have any infections predicted in the next 5 days, as rain is not predicted in the 5-day forecast currently. However, keep an eye on both NEWA and RIMpro scab models to see if the infection prediction changes if the weather forecast changes. With any rain to follow in the future, we will probably have cedar apple rust galls turn from purple brown into the orange, allowing infection of the apple leaves and fruit. Find a nearest red cedar tree to your farm and flag some galls to monitor when the orange spikes will emerge from them. Once they do, make sure you use DMI fungicides in tank mix with mancozeb every 14 days, at least until 2nd cover, as they are the most effective tools to control this disease. Due to the damage on flowers we experienced due to freeze on 8 April and 21 April, the question I get often these days is do we continue investing into fungicide sprays or not?

Spring frosts can be devastating to apple orchards, especially when they occur during bloom or early fruit development. In such scenarios, where no fruit is present after frost, managing diseases like apple scab and powdery mildew becomes critical so as to prevent infection pressure buildup, that carries over into the next year, reduce outbreaks in the current year, and maintain orchard health going into the winter. When late spring frosts wipe out apple blossoms and there is no fruit, some growers think they should leave the orchard fallow that year as it is a season of no harvest, but this will backfire on you and end up with plenty of disease pressure in the following year. Without fruit, management priorities shift toward using cheaper fungicides to preserve tree health, prevent inoculum buildup, and setting the orchard up for success next year. Here’s how to do it effectively:

After a frost, it’s essential to evaluate the extent of damage to the trees first. Look for signs such as blackened or shriveled blossoms, damaged seed house, and calculate the percentage of it across the orchard. If you have an orchard that is a complete loss or with very low amount of crop, make a decision and commit to not use any remaining fruit for commercial use. Share that decision with all farm workers and management, so that no one picks the few remaining fruit later. That way certain fungicides in non-bearing orchards can be used, that otherwise cannot be used if you wanted to use the fruit for consumption or processing. Understanding the severity of frost damage helps in deciding what fungicide spray options you should use. Here are several options: 

​A) If you have not had visible disease symptoms of apple scab, cedar apple rust, powdery mildew or fire blight so far, I commend you, but most importantly do not stop spray applying fungicides and shoot blight control materials (copper, Kudos, Actigard) and keep looking at the apple scab and powdery mildew infection models to apply fungicides just before the major infection periods. This becomes essential if you want to back off on using expensive single site fungicides and stick to the cheaper option of applying contact fungicides like an EBDC mancozeb, phtalimide fungicide Captan, sulfur, and dithiocarbamate ziram.

B) If you decide you will not use systemic fungicides to cut your costs, keep in mind that mancozeb, ziram, sulfur and captan are multi-site fungicides, which are good to use to prevent fungicide resistance occurrence in fungal plant pathogens, but their mode of action is bound only to plant surface – they do not enter inside the green tissue and do not act “on the inside” of it. Therefore, they can be quickly washed off or diluted not only by rain but by leaf tissue expansion (shoot growth) and by alternate row middle spraying on too long of a spray interval. Using the protectant fungicides should be followed at least until after second or third cover to prevent infection of shoots leaves by the usual sprig diseases (scab, rust, powdery mildew).

C) Use RIMpro models for prediction of apple scab and powdery mildew infections to spray multi-site fungicides before the infection periods and secure perfect canopy coverage.  That way if the year is dry, with not that many infection periods, you would spray less – only when the model predicts the infection. On the other hand, in the wet year, the model can tell you that you need to spray more, i.e., increase the frequency of your spray applications. Keep in mind that with multi-site fungicides the perfect coverage matters: the 5-7 days spray interval for alternate row middle will secure good spray coverage, equal to 10-14 day full spray coverage. But you should not apply alternate row middle every 10-14 days in spring as that is not securing good coverage with fungicides. You should also consider frequency of rain: if you get 2-3 rain events per week, even your  5-7 days spray interval for alternate row middle will fail – change your game and in months with a lot of rain events use 5-7 days spray interval for complete sprays (every middle). RIMpro will also tell you when the primary scab season is over, so as to focus on other diseases more.

D) Once you switch to cover sprays, with “First Cover” spray being 10 days after petal fall and when canopy can hold much more fungicide residues due to larger leaf mass, you can switch to complete cover sprays every 14-21 days (every middle), depending on rain, or spray alternate row middle every 10-14 days. If you use sulfur for powdery mildew (Microthiol Disperss) do not use it on the days with temperatures above 80F, as it risks causing phytotoxicity. The every “14-21 days”cover  application rule means applying a cover spray every 14 days or 2 inches of rainfall, whichever came first, whether the 2 inches precipitation was accumulated from a single or multiple smaller rain events. In the absence of rain counting 14 days from the last covers spray, the spray interval can be extended to 21 days unless rainfall occurs during the additional seven days. If a rain event occurred between the 14th and 21st day, the cover must be applied immediately before or after that rain event, irrespective of whether the 21st day was reached or not.

E) Prevention is the best control of plant diseases. So strive to not allow infections on new leaf tissues to happen, but if the infection of scab or powdery mildew do occur, the conidia causing so called secondary scab infections, being released from the primary scab leaf infections, are much harder to control (same for powdery mildew). Even though you will not grow fruit, you still need to control these two disease to prevent high infection pressure to carry over into the next year. Sheet scab is a “special” scab symptom of leaves covered in lesions from secondary scab infections late in spring and early summer by conidia, and are the most conducive to allow overwintering of apple scab fungus when these leaves drop on the orchard floor. As soon as you see any apple scab lesions on leaves, start applying complete spray applications of either Captan 80 WDG at 5 pounds/A or a program of of a Mancozeb 3 lbs/A + Captan 80WDG at 3 lbs. For powdery mildew control, add sulfur (Microthiol Disperss) with at least 10 lbs/A, better 15 lbs/A. Do not use sulfur if you see in the forecast the oncoming days with temperatures above 80F. All these must be reapplied every 7 to 10 days, depending on rainfall and when you reach 1.5-2 inches of rain, reapply these materials as their protective residue is gone. The reason why you need to prevent leaf infections is to allow photosynthesis to go on and nurture the tree and produce food reserves, and ultimately strengthen the tree. Any defoliation of threes prematurely, will weaken the trees and lead to lower number of fruit buds forming for the next year.

F) The goal of bullet E above is that we reduce the impact of the primary scab lesions i.e. to aim to “burn” these lesions on the leaves, thus preventing the secondary scab infections with conidia. If you use captan alone, use full rate of Captan 80WDG at 5lbs/A, but to not apply more then the yearly limit of these fungicides based on the EPA label. Read the label, it’s the law. If you had not had apple scab resistance to dodine, existing lesions can also be “burned out” with Syllit (dodine). Syllit ought to be used at a high rate and tank mixed with an EBDC or captan. Even if you see some powdery mildew, you should use suflur when temperature is lower to reduce infections on terminal and lateral buds formed this for the next year (powdery mildew overwinters under the bud scales).

G) If you do not have fruit on trees, you still can apply the more expensive systemic fungicides, with single-mode of action, in mix with contact fungicides and push out until second or third cover if the rust, scab or powdery mildew have particularly been bad in the last year. Then you can switch to contact fungicides during summer and until leaf drop. However, since the orchard will not give you fruit this year, it is cheaper to use mancozeb alone, which will be effective for rust and scab, and sulfur for powdery mildew.  From 4th cover on, use captan to try and “burn” any scab lesions in the leaves and continue using it until leaf drop because it is effective for bitter rot, Glomerella Leaf spot control, and other rots. The only reason why I am not recommending mancozeb for late season use in non-bearing orchards is because we did not test it that late in the year for bitter rot due to 77 days Preharvest Interval limitation in its use. If you want, you could use ziram as this limit is only 14 days there. The goal here is to protect leaves and buds from infections by Colletotrichum species, and prevent them from overwintering in buds. When fall and winter comes, removing fruit mummies from the canopy, cleaning up the leaf litter and fruit mummies on the orchard floor i.e. chopping leaves in fall, adding nitrogen on the ground would be good to degrade the overwintering sites of Colletotrichum species that cause bitter rot and Glomerella Leaf spot (the use of nitrogen for overwintering pathogen structure degradation was discussed before here: 1. Urea or Lime Can Reduce Overwintering Inoculum of Apple Scab and Marssonina Leaf Blotch in Apple Leaf Litter, 2. Peach Leaf Curl Insurance with Ziram, Copper (Fall) and Chlororthalonil (Spring)).

(H) I, personally, am not in favor of recommending the use of single-site fungicides alone to burn out scab lesions on leaves, in a year when fruit are not present due to frost damage. This might be justified if you had fruit on the trees but they should be always be used at the highest rates and in tank-mix with contact fungicides. We are concerned with single-site fungicides resistance, so use them judiciously and only when apple scab infection models predict the infection.