Apple Scab
Apple scab is a potentially pernicious fungal disease of apples that is endemic to all apple growing regions of the northeastern US. In the simplest terms, the fungal spores (ascospores) of apple scab overwinter on infected leaves. Infected leaves with the spore containing structures (asci) fall to the ground from the tree in late fall. The leaves with asci containing dormant ascospores remain on the ground until the following spring when weather conditions conducive to infections present themselves.

There are several conditions that must exist in order for an infection to occur: 1) receptive green tissue (leaves), 2) susceptible variety, 3) mature inoculum, 4) adequate temperatures, 5) sufficient wetting period. 

1. Receptive green tissue. Early spring scab infections may occur on twigs (unlikely) or flowers/fruit (usually not exposed until early May), but usually only occur on green tissue of rapidly expanding leaves. In any case, if there isn't any green tissue, there won't be a scab infection. 

2. Susceptible Variety. The leaves MUST be of a scab susceptible cultivar (most commercial varieties) like McIntosh or Empire. If the variety is scab immune or resistant the potential for infection is much reduced and control options change. However, scab resistant varieties can vary significantly in their degree of resistance. The severity of any scab infection depends on a variety’s natural resistance and ecological pressures at a given time of year. As well, as we're learning, management style has a profound effect on the susceptibility of any given variety. 

3. Mature Inoculum. The overwintering inoculum is dormant until the ascoposres and asci mature to a point where they are viable for release. Maturity is dependent on weather conditions (largely temperatures) but also depends on the degree of decomposition of the leaves. If the leaves and asci survive the winter and accumulate enough degree days (time x temperature), then all that is needed is a period where the spores can be released into the orchard environment, capable of infecting the season’s newly exposed leaf tissue. But if the leaves are highly decomposed, then the overwintering inoculum's is deprived of its food source and likely in a weakened status come spring.  

4. Adequate Temperatures. Aside from the conditions needed for overwintering spores to mature, released spores that have landed on exposed green tissue need to sit in moisture (rain or dew) for a specific period of time within a specific temperature range in order for the fungal hyphae to grow and infect the tissue. Depending on the length of the wetting period, an infection can occur over a fairly wide range of temperatures. Infections during shorter wetting periods (9 hours) need temperatures in the range of 65-72F for a significant infection period to occur. Longer wetting periods can result at higher or lower (but less optimal)  temperatures on either side of that range up to certain points. 

5. Sufficient Wetting Periods. Above all else, an apple scab infection needs adequate free moisture for a minimal amount of time (temperature dependent) for an infection to occur. Moisture can be in the form of rainfall, dew, sprayer droplets, frost, snow, etc., but it needs moisture. High humidity will not suffice.

Mills Chart.
The Mills Chart was devised to illustrate the conditions required for apple scab infections under various conditions. There is a wide range of temperatures and wetting periods that can result in either high, medium, or low infections. Assuming that you have susceptible green tissue and viable inoculum, the rest is left up to Mother Nature.

More recently the use of computer models have allowed for researchers and growers to have a better understanding of how scab develops and when to expect a significant infection period. The use of Cornell's NEWA site and the European RIMpro now provide a level of detail and sophistication that is just beginning to be accepted by growers. In addition to the basics also used by the Mills chart, these two models can prdict (based on degree days and weather forecast) how the scab organism (ascospores) is maturing, how many are released with each wetting event, and how many remain over time. This puts a real beginning and end to each primary scab season that should give growers a confidence of when to spray and when they can go back to playing pinochle. 

Primary vs. Secondary. The last point to make is exposure to scab infections doesn’t need to last all year long. Early season infections are called primary infections. Infections that occur later in the year result from the early season infections and are called secondary infections. Primary infections largely infect leaves, while secondary infections largely infect fruit and leaves. So, if you can control the primary scab infections from overwintering inoculum you will have fewer problems later in the season with secondary scab. Primary scab season extends green tip until the middle of June (roughly. 

Orchard sanitation. Unless you have planted an immune variety, then even moderately resistant varieties can become infected under the right conditions. The best way to prevent even mild infections is to make sure you don’t have any inoculum (or you have very little) by keeping your orchard clean. There are a variety of ways to do this, but anything that helps with the decomposition process only works in your favor. The best known way is to spray trees with a combination of urea, copper, and/or lime sulfur. The copper and lime sulfur kill any surviving scab fungus, while the urea helps to decompose the laves so the food source is gone long before spring gets there. 

What to Spray?
There are many options out there. Organically, none are 100% slam dunks, but we are developing better ways to control scab without using harmful fungicides. The following materials are some of the best options available