Frog-eye / Black Rot of apples and pears

Need to know: 

• Symptoms usually begin as small purple spots and grow to resemble a frog’s eye.
• The fungus can enter through natural openings, wounds, and injuries on the trucnk and branches. 
• Spores are spread by wind, splashing water, and insects. 
• Manage by scouting and pruning. Preventative fungicides can be used rotationally.  

Overview

Frog-eye or Black Rot is a disease caused by the fungus Botryosphaeria obtuse, in apples and pears. This pathogen can infect all parts of the tree in fruits, leaves, branches, and trunks. Its symptoms may resemble fire blight.

This disease can cause severe symptoms on both the fruit and the tree woody tissue. If not properly managed, disease will worsen year to year and impact home and commercial orchards.

Symptoms of Frog-eye/Black Rot of apples and pears

An early symptom of frog eye black rot is the formation of small purple flecks on leaves that enlarge to roughly 4-5mm in diameter. The margin of the lesion remains purple while the center turns tan or brown in color giving it a frog-eye appearance.

Leaf symptoms include leaf spots that are circular and brown with tan interiors. They begin as small purple spots and eventually grow to resemble a frog’s eye, hence the disease name. These are usually not as serious unless, in rare cases, the leaves begin to turn yellow and fall off of the tree.

The fungus causes sunken spots (cankers) with enlarged margins on the bark of woody tissues. Cankers can form on the limbs of infected trees appearing reddish or pinkish brown in color and sunken into the bark. These spots on the bark can resemble fire blight cankerss. Look for these cankers at the crotches of branches and twigs where they meet the tree's trunk.

Brown rotting may occurs on the blossom end of the fruit with only one spot, occasionally with concentric rings as infection spreads, that eventually turn black. Fruit may mummify and remain on the tree.

Signs of Frog-eye/ Black Rot of apples and pears

Small black structures (pycnidia) may be observed on infected fruit late in the season or while fruit is being held in storage. Microscopic Fungal bodies on sunken areas may be evident upon inspection under the microscope.

Not to be confused with Fire Blight

Disease Cycle

The fungus can enter through wounds on fruit, stomata, and injuries on the trunk and branches. The disease occurs with infection from ascospores and conidia released from pycnidia.. They are released all throughout the season. Spores are spread by wind, splashing water such as in rainfall, and insects which act as vectors for the disease. Ideal conditions for spreading of the disease occur between 48 - 80 °F.

Type of sample needed for confirmation/diagnosis

The Iowa State University Plant & Insect Diagnostic Clinic can help you investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa, please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us.

Want to submit a sample?  Follow the foliar instructions on the tree and small fruits page

Management

Scouting. When scouting early in the season, look for small purple spots on the leaf surface that eventually grow to have larger brown interiors. If you spot mummified fruit, this is a key sign that the disease has developed. Look at branch intersections with the trunk for enlarged cankers on trees with more development of the disease. Sunken spots can also be seen on branches.

Physical. Prune dead or diseased branches carefully, sanitizing tools between cuts as openings can act as sites for disease transmission. Pick fruit, either mummified or at earlier stages, and discard. Burn or bury infected tissues after they have been removed from the site. If cutting down any trees, remove stumps as they can be sources for spores.

Chemical. Homeowners can use full-rate sprays in the early season with copper-based products, lime-sulfer, or Daconil. In commercial settings, many fungicides can be use in rotation  part of a spray program, for more information see the Midwest fruit Guide.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.

By Taylor Mauch (ISU Horticulture Graduate student) and Lina Rodriguez Salamanca