January 15, 2026

(I) Urea or Lime for Reducing Overwintering Inoculum of Apple Scab and Marssonina Leaf Blotch; (II) Peach Leaf Curl Control with Ziram and Chlororthalonil in Spring

(I) Reducing Overwintering Inoculum of Apple Scab Fungus (Venturia inaequalis) and Marssonina Leaf and Fruit Blotch (Diplocarpon coronariae)

If symptoms of apple scab were visible on leaves or fruit during the 2025 growing season, you should plan to reduce overwintering scab inoculum residing in leaf litter on the orchard floor. What you do now for inoculum reduction  can increase the efficacy of your apple scab fungicides in 2026 growing season. The most conducive for apple scab fungus overwintering are numerous late summer and fall infections on leaves visible as small lesions on the underside. We call these “sheet” scab (Figure 1).

Figure 1. Late summer and fall scab infections on the underside of apple leaves (Photo by S. G. Acimovic, 2009).

The late season/fall scab lesions are the most productive places where apple scab fungus V. inaequalis will grow into the internal tissue of dead leaves after on the orchard floor and form initials of pear-like fruiting bodies of the fungus called pseudothecia (Figure 2). These pseudothecia will facilitate release of apple scab ascospores in the spring of 2026 with each wetting event, starting from green tip growth stage of apples.

Figure 2. Round-like pseudothecium of apple scab fungus harbored in the dead leaf tissue visible under the microscope. Long sacs with ascospores called asci are visible as fingers protruding outside from the squashed pseudothecium (Photo by S. G. Acimovic, 2007).

Pseudothecia are protected inside the dead apple leaves on the orchard floor and when they mature in spring they release ascospores (Figure 3). Ascospores cause first infections on green apple bud tissues in the spring impacting leaves, fruit and petioles of both leaves and fruit. If you had a high disease pressure last year, you can reduce overwintering inoculum dose in leaf litter by applying urea and/or lime at this time of the year (fall, but also in spring, or both in spring and fall). The applied urea or lime help microorganisms and worms in and on the soil to quickly degrade leaf litter protecting pseudothecia. Alternatively, when there is no snow cover and it is not muddy, you can use the flail mower to shred the dead leaves to smaller pieces – that will help speed up the degradation of leaf litter by these organisms. In smaller orchards, you can rake the leaves under the trees into the row middles and remove leaf piles with flail mower mode for scalping the sod (Cox, 2016). However, the most used option is to spray apple leaves with urea just 1 to 2 days before the major leaf drop in fall (first severe frost) when leaves are still on tree branches. This will secure better coverage of leaves with urea. If you missed to do this, you can apply urea to leaf litter on the orchard floor. Use the rate of 40 lbs of urea / acre in 100 gals of water. Make sure this practice is done late enough in fall and start of cold weather to safely avoid promoting any late growth with the spray applied nitrogen. If you aim to spray leaf litter on the ground, the best timing to do this is late winter but before bud break. Turn the airblast air deflectors downward and turn off top nozzles to allow spray mist to lift the loose leaves on the orchard floor and coat them with urea as much as possible. After application, always 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.

Figure 3. Bullet-like ascospores of apple scab fungus (center right) with asci sacs containing still un-ejected ascospores in behind on the left (visible under a microscope) (Photo by S. G. Acimovic, 2007).

Instead of urea, and many times this is the only option in organic orchards, lime can be applied at a rate of 2.5 tons / Acre (Spotts et al. 1997). If lime is used, it is better to use it after the leaf drop in fall or early in the winter. Lime increases pH or basicity (opposite to acidity) promoting microbial activity and leaf litter breakdown. Use powdered lime form dispersed by lime or fertilizer spreader as the most efficacious way of applying it.Reduction of ascospore inoculum dose with these two methods can range from 50 – 70% (MacHardy et al. 2001; Sutton et al. 2000). Reduction of scab inoculum using the above described flail mower, urea and/or lime applications does not mean you do not need to spray against apple scab next spring. Enough apple scab inoculum will always survive and be present in the orchard or neighboring orchards to cause infections if gaps in fungicide protection are allowed. Inoculum reduction applied during dormancy actually secures better efficacy of your fungicide spray applications in 2026 growing season and reduces overall chance for severe scab infections.Marssonina leaf blotch (MLB) is an emerging apple disease in the Eastern Coast. It requires wet summer to infect and leads to crown defoliation and fruit spots (Figures 4, 5). It is caused by the fungus Diplocarpon coronariae. You can read more about this disease by opening this link: Apple Blotch Disease. MLB defoliation can range between 20 – 70% usually occurring from September to October. Early fall loss of canopy could expose fruit to sun and lead to sunburn, but the added concern are infections of fruit that can happen on trees with significantly defoliated crown and can carry over into the storage (we have seen this often in organic orchards). Cumulatively, with years of tree crown defoliation, apple trees can set fewer buds and weaken due to premature loss of leaf mass to this disease. This reduces accumulation of starch reserves in the trunk and scaffolds and can push more green growth late in the fall when the trees should be preparing for dormancy. The primary host for D. coronariae is apple (Malus domestica) but other decorative species such as Malus baccata and Chaenomeles spp. are also known hosts of this fungus and serve as inoculum sources in the landscape.

Figure 4. Typical late season Marssonina Leaf Blotch symptoms on apple leaves.
Figure 5. (A-D) Progression of Marssonina Leaf Blotch symptoms on leaves and (E, F) infections on fruit (cv. Rome).

MLB fungus D. coronariae overwinters in fallen leaves on the orchard floor (EPPO 2013). Ascospores originating from overwintered fallen leaves cause first infections in spring and summer. They can form in the overwintered cup-like fungal structures called apothecia that form on the leaf litter from last year, but it is not known whether the apothecia and Diplocarpon ascospores can form in Virginia weather conditions. It has been reported that in regions where apothecia do not form, the fungus overwinters as asexual spores (conidia) on fallen leaves (Back and Jung 2014). It is highly likely that this is happening in Virginia. Nevertheless, ascospores still might form and could serve as inoculum for primary infections. Conidia are asexual spores produced in plate-like bodies on the leaf surface, called acervuli. Acervuli are spore groups visible as small black round specks after leaf epidermis is ruptured by pushing the spore masses out. You can see them as black dots inside the brown/black blotches which can be seen with a naked eye or a magnifying glass. In the case that ascopores do not form, conidia infect leaves first in mid- to late-spring (Figure 6). Conidia can cause multiple secondary infections during the season. RIMpro, which is accessible online at https://rimpro.eu/ offers an MLB disease prediction model that allows to predict when will the first infection of the season will occur based on weather forecast. The model will indicate to you when to start your first spray before the first infection is predicted. It seems that in Eastern U.S., infections during very rainy summers start late in May or the first week of June. Typical symptoms are usually visible 40 – 45 days after the infection (Lee et al. 2011). In the orchard, MLB spores disseminate by rain and wind. Trade with nursery material that carry infected leaves allows introduction of this disease into the new regions.

The inoculum of MLB can be reduced by orchard sanitation cultural practices. Shredding the fallen leaves on the orchard floor which serve as sources of inoculum, by flail mower or raking and burning the leaves, can reduce the spore dose next spring. Application of 40 lbs of urea/A in 100 gals of water onto the overwinter leaves and raking the leaves from under the trees for later shredding by flail mower can reduce the inoculum dose of this disease. If urea is not an option (organic orchards), application of dolomitic lime (2.5 tons per acre) just before leaf drop in fall or early in the winter can help apple leaf litter breakdown and thus aids to the reduction of the spore inoculum dose in the overwintering leaves (this rate can also be used in apple orchards with synthetic fungicide programs).

 

Figure 6. Typical spores of the fungus Diplocarpon coronariae which causes Marssonina Leaf and Fruit Blotch visible under the microscope and are situated in acervuli – the small black dots you see in the brown patches of leaves collected in Figure 4 (Photo by Aćimovic S. G. 2019).


(I) Dormant and Spring Fungicide Options For Peach Leaf Curl (Taphrina deformans)

Copper for leaf curl should be applied while trees are dormant. Many folks wait for the leaves to drop and apply it in early winter. In most years, due to unpredictability of weather in spring, which can create muddy mid-rows, the best time to apply peach leaf curl copper is late in winter and up to and during bud swell. Use copper material of your choice delivering at least 4 to 8 lb of metallic copper per acre. Ziram for peach leaf curl should be used during dormancy, after leaf drop in fall, and prior to bud swell. You can combine Ziram and copper or Bravo (chlorothalonil) and copper. Ziram and copper can be applied up to bud swell but before bud break. Copper aims to reduce the spores of peach leaf curl fungus T. deformans which overwinter on the tree buds. Infections take place in the spring as the buds open. This fungal pathogen infects buds during rain events from bud swell to bud opening. So if spring is rainy and cold, and buds open slowly, fungus has more chance to infect and your application in fall will pay off as you will not be able to get into the orchard due to muddy mid-rows.Very long cool wet periods during bud burst can slow peach bud development and thus lead to severe peach curl infections. Copper products also give some suppression of bacterial spot as well (Xanthomonas arboricola pv. pruni). Here is more info on copper against this bacterium: https://dev-ppa.pantheonsite.io/copper-bactericides-for-peach-bacterial-spot-management/ In spring, if you missed the window for late winter copper application, 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 and Bravo and its generics redistribute well during rain.

LITERATURE

Lee et al. 2011: Biological Characterization of Marssonina coronaria Associated with Apple Blotch Disease. Mycobiology. 39: 200–205.

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Filed Under: 2026 GROWING SEASON