1. First Apple Scab and Cedar Apple Rust Symptoms on Leaves Visible in Winchester on 16 May; 2. Primary Scab Season is Over on 10 May

1. First Apple Scab and Cedar Apple Rust Symptoms on Leaves Visible in Winchester on 16 May; 2. Primary Scab Season is Over on 10 May

All the major apple fungal diseases in the Shenandoah Valley are visible in 2023 growing season: apple scab (Figs 1, 2), powdery mildew (previous post from 8 May), and rust (Fig. 4). On 16 May, I found first fresh scab infections on ‘Gala’ leaves (Fig. 1) at AREC. These symptoms were fond on apple blocks not protected by fungicides. When the scab lesions were examined with microscope, asexual spores of apple scab fungus called conidia were detected and will facilitate secondary infections (Fig 2). First dry and warm weather in the first half of spring and then cold and wet weather in the second half of spring 2023, has slowed down disease symptom occurrence. In terms of scab, we established two orchard blocks in March 2023 that have a high ascospore dose pressure here at AREC. We have secured this pressure by introducing infected leaf litter from the 2022 growing season. One of the first impressions when I saw scab visible as dark olive green spots (Fig. 1), was that infections were NOT on the apple spur leaves, which develop first in the season, but on the younger leaves developed mid-spring. This correlates well with the apple scab RIMpro model reporting that the first major infection of the season occurred on 15 April (Fig. 3).

Figure 1. First apple scab symptoms on leaves of ‘Gala’ in Winchester VA at AREC experiment orchard on mid-Spring developed leaves (Photo by Acimovic S. G. 2023).


Figure 2. Asexual spores of apple scab fungus detected under microscope from the scab lesions on leaves in Figure 1 (Photo by Acimovic S. G. 2023).

Based on RIMpro model, primary apple scab season has ended in Northern Virginia on 10 May. Regardless of the primary scab season end, the fungicide coverage needs to be continued on a tight spray interval up to 2 weeks after the date of primary scab season end (10 May 2023, for Winchester). This is absolutely needed because you do not know how good your coverage was in the major spring scab infection events and rust infections warrant continued applications ahead. Many of you are using alternate row middle (ARM) spraying that should cease at bloom and you should spray every row from bloom onward. So, if any scab lesions arise now from coverage issues associated with ARM spray approach, or due to missed fungicide applications before any rains during early spring, it will be much harder to have perfect looking fruit crop until harvest. Risks from scab are now high in orchards where scab lesions are visible on leaves or on fruit. This is because asexual spores of apple scab fungus called conidia are available for infection (Fig 2). In those orchards protection from scab scab continues with higher rates of captan as the secondary  infections by asexual spores called conidia, which is what is inside of the scab lesions you see, and are now able to infect. If you see scab lesions let me know. Leaves are still expanding now on terminal shoots and can hold much more fungicide residues to protect from diseases, allowing some redistribution during low-amount rains.


Figure 3. RIMpro apple scab model output for 2023 in Winchester, VA (historical data from a weather station). White camel hump-like areas labelled “Germinating spores” show cumulative number of Venturia inaequalis ascospores that germinate over time and are read using the right-side vertical Y-axis scale that is labelled “Discharge”. The red curved lines are scab infections. Read each red curve’s peak RIM infection value(s) using the vertical Y-axis scale on the left-hand side of the graph labelled “RIM Infection Value”. Peak RIM value divided by 100 gives you the percentage of the total season’s ascospores that will cause infection for a given infection period (wetting event). Orange area labeled “Primary stroma” represents scab lesions that were initiated by infection from germinating spores and that are incubating in the leaf after which scab lesions will become visible. This is worth knowing because if no fungicide was applied before the infection started, some or all of the incubating infections can be eliminated by using fungicides with post-infection activity. The light red area in the middle graph labeled “Maturation” is the proportion of mature ascospores that are ready for discharge with wetting events whereas the dark red area shows the proportion of immature ascospores still remaining in leaf litter on the orchard floor. The dark blue bars in the bottom graph showing wetting and dates, are the actual rain periods. The light blue bars are actual wetting periods when no rain is falling but trees are still wet after rain. Courtesy of RIMpro B.V., France.



Figure 4. First rust infections on ‘Fuji’ leaves in Winchester VA (Photo by Acimovic S. G. 2023)

On 16 May, we also detected first cedar apple rust infections on ‘Fuji’ leaves (Fig. 2). Based on the look of the galls on Juniper trees (Fig. 5) it seems that the telial horns as source of basidiospores are depleted and no longer a threat for causing infections.

Figure 5. Depleted galls of cedar apple rust fungus Gymnosporangium juniperi-virginianae (Photot by Acimovic S. G. 2023).