Avocado Black Spot: Learn About Cercospora Spot In Avocados
By: Kristi Waterworth
There are lots of great things about living in a warm climate, but one of the best is being able to grow amazing fruits like avocado in your own backyard. Growing more exotic plants can be both a blessing and a bit of a curse, though, because this also means that you have fewer resources to help when you run into a problem. For example, if you notice that your avocados are developing weird spots, you might get a little suspicious. Could it be avocado black spot, more commonly known as cercospora spot in avocados? Read on for a more in-depth discussion of this chronic disease of avocados.
What is Avocado Cercospora Spot?
Avocado cercospora spot is a common and frustrating fungus that thrives on the tissues of avocado trees. The disease is caused by the pathogenic fungus Cercospora purpurea, but it presents much like other types of Cercospora infections. Cercospora symptoms can include, but are not limited to, small brown to purple spots on leaves, angular-appearing spots on leaves, small irregular brown spots on fruits or fissures and cracks in the fruit’s surface.
C. purpurea is spread by wind and rain, but it may also be transmitted by insect activity. Fruits tend to become infected during the wettest part of their growing season. By itself, Cercospora won’t damage avocados beyond use and the fungus doesn’t penetrate the rind of the fruit, but the fissures that can result from the fungal feeding invite more destructive pathogens into the flesh.
Treating Avocado Cercospora Spot
The goal of any avocado grower should be to prevent fungal diseases like Cercospora spot from erupting in the first place, so before you consider treatment, let’s talk about prevention. Cercospora is often transmitted from plant debris or weeds that are around the tree, so make sure that you clean up all fallen leaves, shed fruit, and keep the area free of unwanted plants. If there are any avocados that didn’t get picked and didn’t fall last year, get those things off the tree ASAP.
The other part of the equation is airflow. Fungal infections love pockets of stagnant air because they allow humidity to build, creating a fungal nursery. Thinning the inside branches of your avocado, like with any fruit-bearing tree, will not only decrease the humidity in the canopy, but also improve the quality of the fruits you get. Sure, you may get fewer fruits, but they’ll be significantly better.
The actual treatment of Cercospora is pretty straightforward. Copper spray, applied three to four times a year, seems to keep the fungus at bay. You’ll want to apply the first at the beginning of your wet season, then follow up monthly. The third and fourth are only recommended for avocados that ripen very late.
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(View photos to identify causes of fruit damage)
Sunblotch causes a wide variety of symptoms or may exhibit no symptoms in some hosts. Symptoms of sunblotch include necrotic, red, yellow, or white discolorations on fruit, often in depressions or scars in the fruit surface. Twigs can develop narrow, necrotic, red or yellow streaks on their surface or in shallow lengthwise indentations along the twig. Leaves may have white or yellowish variegated areas and be deformed, but leaf symptoms are uncommon. Rectangular cracking and checking of the bark, called "alligator bark," often occurs on the trunk and larger branches. Infected trees may be stunted and have a disproportionate amount of horizontal growth or sprawling lateral low limbs. Trees with visible sunblotch symptoms often have reduced yields. Infected trees can also be symptomless, although large reductions in yield of previously vigorous trees may indicate the presence of the viroid in otherwise symptomless carriers.
Pests and diseases of avocado crops
Root rot and anthracnose are the 2 main diseases that affect the Queensland avocado industry. Fruitspotting bug is the main pest.
Phytophthora root rot
Avocado trees are very susceptible to root rot, which is caused by the soil-borne fungus Phytophthora cinnamomi. Controlling this disease is the highest priority for avocado growers in Queensland. It is ever-present and requires constant attention. Without root rot management the tree will lack an adequate root system and can’t perform.
Effective root rot control comes from an integrated approach that includes:
- disease free nursery trees
- using more tolerant rootstocks
- good drainage which includes the use of tree mounds
- mulching and good soil health
- appropriate soil moisture management
- chemical treatment
- good nutritional practices including sufficient calcium
- suitable soil pH.
Chemical root rot treatment
Phosphonate fungicide is the best chemical root rot treatment, both as a regular preventative treatment and to cure the disease. It is essential that you maintain an annual, or in some cases biannual, treatment with phosphonate. You can apply the fungicide to the leaves (only if the trees are healthy) or inject it directly into the tree by trunk injection. If trees are too small to inject with phosphonate fungicide, you can use foliar sprays or metalaxyl granules on the ground. Seek specialist advice on the correct use of phosphonate because timing, rates and method are critical.
Replanting after root rot
It is difficult to get a replant tree to grow where the original tree has died from root rot. If the trees either side of the gap won’t effectively colonise the space, follow these steps.
- Select a planting site at least 2m away from the site of the dead tree and prepare the site for replanting.
- Dig it over, check the pH and apply lime or dolomite and organic manure as required.
- Mulch the site and leave for at least 3-6 months (or more if possible) before planting.
- Apply metalaxyl granules around the tree at planting and again 8 weeks later.
- Adjust the irrigation emitter and fertiliser rate for the smaller size of the newly planted tree.
Anthracnose is caused by the fungus Colletotrichum gloeosporioides and presents as a fruit rot that remains latent until the fruit ripens. Fruit infected with anthracnose can rarely be identified at packing and is usually only discovered at the point of preparation by the consumer.
Just like other varieties, Hass is vulnerable to anthracnose but difficult to detect until it is cut because its thick dark skin masks the symptoms.
Management includes regular orchard sprays (every 2-4 weeks depending on weather conditions) from shortly after fruitset until harvest using a registered protectant fungicide such as copper oxychloride.
Fruit-spotting bug and banana-spotting bug
Fruit-spotting bug and banana-spotting bug have a similar effect and either 1 or both species are found in all Queensland avocado growing areas. These are serious pests which sting the fruit at all stages from fruitset until picking.
- fruit shedding (if stung when small)
- fruit distortion and dimpling
- woody ‘stones’ where stung.
The pests have hundreds of alternate hosts and are very mobile so can quickly re-infest after a spray. They are also quite secretive and will move out of sight when you approach so are difficult to spot. A fresh batch will often fly into an orchard on a hot windy day.
By observing where damage is worst you may be able to identify ‘hot spots’ in your orchard which can be used for subsequent monitoring. Regular sprays of a registered insecticide are generally needed but once you become confident with monitoring it may sometimes only be necessary to spray the hotspots.
Learn more about prevention and treatment for fruit-spotting bug and banana-spotting bug .
Ecological Requirements for growing avocados
Although avocado is relatively resistant to drought, it needs a well-distributed rainfall of between 1000-1200mm. This is enough for crop development with a two-month drier season for pre-flowering. Most avocados need irrigating. Your plants require about 25 mm of water per week. It would help if your plants also had an optimum temperature of about 20-24 degrees Celsius for sufficient growth.
Moreover, you require well-drained soils to avoid root rot. The best soils are alluvial, sandy or loam soils with pH ranging from 5-7. The Kenyan climate is sunny, the soils are dry and contain proper drainage, suitable habitat for avocado trees.
How to Treat Entomosporium Leaf Spot
Entomosporium leaf spot is a fungal disease you definitely want to avoid. Once leaves are infected with the disease, there is no saving them. You can, though, save your plant -- that is, if you catch it early and act immediately. Shrubs and trees that are members of the Rosaceae family, subfamily Pomoideae, are susceptible to entomosporium leaf spot. These include plants such as hawthorns, quince, photinias, pears and flowering crab apples. The disease thrives and spreads under moist, humid conditions. The way to treat entomosporium is to get rid of the affected leaves to prevent spread and to prevent it from returning.
Remove the affected leaves and discard in plastic bags in the trash can. The disease usually starts at the bottom of the plant and works its way up. Look for tiny red dots that sometimes have a yellow ring around them. The spots darken and get larger over time.
Clean up the leaves on the ground. These may also have entomosporium or be susceptible to it, as fallen leaves tend to stay wet for long periods of time. Discard in the trash, not in your compost pile.
Prune lower branches to provide at least 6 to 12 inches of space between trees and shrubs and the ground. Thin out the center of a shrub or tree to increase air flow. If plants are grouped closely together, remove some of them or prune them to maintain at least 12 inches of space between them.
Remove mulch and other plants growing beneath affected trees and shrubs to increase air flow to the affected plants. This also reduces moisture that accumulates at the base, decreasing humidity around the plants.
Treat the tree with a fungicide, such as chlorothalonil. Apply when conditions become moist for several days, such as after a rainy period or high humidity. Spray it on both sides of the leaves, per label instructions. This helps prevent entomosporium from returning.
Water all plants susceptible to a fungal disease at ground level, not from overhead. Water them in the morning so that the area has time to dry out before nightfall.
Melissa Lewis is a former elementary classroom teacher and media specialist. She has also written for various online publications. Lewis holds a Bachelor of Arts in psychology from the University of Maryland Baltimore County.