By: Amy Grant
Avocados are a delicious, healthful fruit that, like all crops, may become afflicted with a disease. Avocado scab disease is one such problem. While initially scab on avocado fruit is a cosmetic issue, it may become a gateway for the entry of fruit rotting organisms such as anthracnose. Identifying scab symptoms in avocado will better enable the grower to apply avocado scab control.
Avocado scab disease is caused by the fungus Sphaceloma perseae. Scab symptoms on avocadoes present as oval to round raised areas of corky scab. The first lesions that appear are generally black/brown and scattered across the fruit’s skin. The lesions begin to coalesce and merge, potentially affecting almost the entirety of the fruit.
Symptoms of scab on the leaves are more difficult to ascertain, as the most visible signs are in the uppermost portions of the tree’s canopy. Young leaves may become distorted and stunted with reddish spots on both the upper and lower sides of the foliage.
Scab symptoms on avocado may be confused with physical damage. Fruit is most susceptible right after fruit set and during the early stages of development. When the fruit is at about half its mature size, it becomes resistant to the infection, as do leaves once they are about a month old. The disease is most prevalent after long periods of rain, especially when the tree is in its initial stages of fruit set.
Although the disease is primarily cosmetic, affecting the exterior of the fruit but not the interior, it is a portal for other diseases, so treating avocado scab before any sign of infection is necessary to the health of the tree and resulting fruit. Also, since scab is spread by the dispersal of spores produced at the early stages of infection and then spread through the movement of wind, rain, and tools or equipment, the pathogen can travel over long distances.
Fungicides should be used to mitigate the spread of the fungus. Treatment involves the application of copper fungicide when flower buds appear, near the end of bloom time and again 3-4 weeks after.
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Careful propagation of nursery stock to eliminate viroid has greatly reduced sunblotch to a relatively minor disease. However, ongoing monitoring and management is required in nurseries and established groves. Sunblotch is easily overlooked, and there are many ways that trees can become infected. Look for disease and disease-promoting conditions regularly throughout the grove by MONITORING DISEASES AND DISEASE-PROMOTING CONDITIONS.
The danger of spreading viroid increases in established orchards where mature trees are pruned to reduce tree size and re-stimulate or maintain fruit production. Severe pruning of symptomless carriers, and perhaps other severe causes of tree stress, are suspected of causing viroid to become active in the new growth, inducing previously symptomless trees to exhibit symptoms. Disinfect pruning tools, harvest clippers, and injection equipment before beginning work on a new tree. Scrubbing tools clean and then soaking them in a 1.5% sodium hypochlorite solution is effective. Growers must use a registered disinfectant and follow label directions.