Proactive versus reactive disease and insect management in homegrown apples

apple on branch with scab-like growth on it, next to leaf with similar scab-like spots
Fig. 1. Fungicides and insecticides will not cure infections that have already caused damage. Yonghao Li, The Connecticut Agricultural Experiment Station,

While the apple tree is claimed to have helped Isaac Newton understand gravity, the old saying ‘“an ounce of prevention is worth a pound of cure” has been attributed to Benjamin Franklin is likely the most appliable quote in regards to effective apple pest management. 


While scouting for insects and diseases is very important, management for pest issues begins well before symptoms and defects appear (Figure 1).  Populations of insect pests can be scouted for, however the challenge with fungal diseases is by the time we see physical signs and symptoms on plants, its generally too late for effective disease management. 




Fungicide considerations 

Most of the fungicides available to the general public (and most fungicides in general) do not control disease by killing existing infections (i.e. curative), but rather by preventing new infections (i.e. protectant).  Fungicides, like captan, mancozeb, etc., are what we referred to was protectant fungicides. Protectant fungicides do not get absorbed systemically, i.e. do not move throughout the  plant.  These types of products only prevent new infections from forming, and do not control existing infections (Figure 2) 

gradient graph showing efficacy of fungicides varying depending on timing
Figure 2: Theoretical control of fungal diseases by protectant fungicides.  Infections that occurred prior to spraying (red, left side) are minimally controlled (<20%) by these types of fungicides, while infections that occurred a week or more after the application are controlled very well (green area), but eventually the product loses effectiveness (red, right side).  Note that infections can take several weeks to see, so once you see an infection, its likely it actually started 1-3 weeks ago. 

Most fungal pathogens require moisture to infect, and Venturia inequalis, the pathogen that causes apple scab, is no exception.  We know that apple scab is most likely to spread in the early spring when the leaves and flowers are emerging, and the risks decline after petal fall.  So, applications should be made during that time frame.  If we delay the applications, the pest will build up and create larger issues. 

Often, there are questions about when to apply fungicides in regards to precipitation.  There are a number of products that become absorbed by the trees and remain effective after precipitation, but the protectant products only work on the surface of the leaves.  So, after a heavy rain event, they are often washed off.  Due to this, many people ask if they should wait to apply these products after the rain event.  While we don’t want to apply these products only an hour or so before a rain event, remember that they only work by preventing infections.  Since precipitation is what allows the fungus to spread, for these products to work, they need to be applied well before a rain event. 

For the commonly used protectant fungicides, its generally thought that they can remain effective 10-14 days after application.  Keep in mind though, during the time of year with rapid shoot growth, the new leaves that emerged since the last application are not protected.  While there are a lot of factors that go into whether or not a protectant fungicide was washed off in a rain event, some generalities are that in ½-1” rain, the products have lost some effectiveness and may need to be reapplied (Figure 3). For more on apple disease spraying, see this Extension article. 

gradient showing how rainfall post application affects efficacy of fungicide application
Figure 3: Example from Figure 2, except a rain fall event post application has reduced the effectiveness of the fungicide and may require an additional application. Note that the fungicide ‘did its job’ by being on the plant prior to the rain event, and so prevented significant infections from occurring.

Don’t forget about Integrated Pest Management Practices 

In general, managing for disease and insect issues begins with good IPM practices. Each apple cultivar has its own inherent level of disease resistance or susceptibilities.   

There are other cultural management tools that can be utilized to limit pest pressure in subsequent years. Namely, sanitation. For example, if your apple tree is plagued by apple scab this year, raking and destroying all of the fallen leaves can help limit the disease problem next year. For insects, while management practices can differ depending on the pest, you can be proactive about management to get ahead of potential pest issues. Monitor your apples regularly throughout the growing season for the presence of insects on fruits. If any apples are experiencing insect-related damage, it is best to remove the fruit and clean up any apples that fall from the tree.  


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 23, 2024. The information contained within may not be the most current and accurate depending on when it is accessed.