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Resistance does not equal immunity

M. Grabowski, UMN Extension
Crabapple 'Adams' has good resistance to apple scab but has
some leaf spots this year. M. Grabowski, UMN Extension

Planting disease resistant plants is a great way to reduce disease problems in the garden without pesticides or the added time and labor needed for many cultural control practices. A disease resistant plant is able to defend itself against a plant pathogen. In some cases, a resistant plant is very successful in its defense and the gardener will not see any visible symptoms of disease. In other cases the plant may develop low levels of disease but is able to slow the pathogen and prevent severe damage from disease. These plants may be marketed as disease tolerant, moderately resistant, or resistant.

This year, weather conditions have been highly favorable for apple scab, a fungal disease of apple and crabapple trees. The apple scab fungus infects both leaves and fruit. Leaves have olive gray to black spots with a feathery undefined margin. Severely infected leaves turn yellow and fall off mid-summer. This year, due to weather conditions favorable for apple scab, many susceptible varieties of apple and crabapple have thin see through canopies with small piles infected leaf debris collecting on the ground below.

Severe apple scab leaf infection on a
crabapple variety susceptible to apple scab.
M. Grabowski, UMN Extension 
Gardeners may be surprised to see that even apple and crabapple varieties that are resistant to apple scab have some leaf spots and even some leaves turning yellow. It is important to understand that these disease resistant trees have not lost their resistance to apple scab; they are simply fighting against a strong and thriving pathogen population this year. Despite the presence of a few leaf spots, apple scab resistant varieties have thick full green canopies and very little leaf loss. This means that the stress placed on the apple scab resistant varieties is significantly less than the stress on the defoliated susceptible varieties.

Full canopy on crabapple variety ' Adams' despite some apple scab.      
M. Grabowski, UMN Extension 

Apple scab susceptible crab apple variety with
a thin see through canopy due to leaf loss.
M. Grabowski, UMN Extension

For a list of apple scab resistant crabapple varieties and more information about management of apple scab read the UMN Extension publication on apple scab

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