Marty asks: My avocado trees seem to be doing well here in Waimea but after I pick the fruit black spots appear and the avos seem to rot really quickly. What could this be and is there anything I can do about this problem?
Tropical Gardener answer: The symptoms you describe sound like those of the fungal disease anthracnose. Though the avocados may look fine on the tree, a latent fungal infection can become symptomatic once the anti-fungal compounds in unripe fruit diminish as it ripens.
The pathogen, Colletotrichum gloeosporioides, creates dark, sunken spots that spread rapidly on the fruit’s skin and eventually begin to rot the fruit itself. Obviously, this disease reduces avocado’s shelf life and thus its marketability.
If you are eating or gifting most of the fruit, the good news is that anthracnose infected fruit is safe to eat. The taste around the rotting area is usually unpleasant, however. Since the fungus spreads quickly in the fruit, you should eat it before or soon after you see the black spots developing.
As with most plant pests, prevention is the best treatment. Anthracnose thrives in moist conditions. Try to plant avocado varieties that ripen during dry seasons of the year. Some avocado varieties are less susceptible than others so try to plant those that have some anthracnose resistance.
Thin out internal branches so that wind can pass through the canopy without breaking branches. Anthracnose spores often lie dormant on branches and leaves when no fruit is present. Broken branches can create vulnerable holes in fruit and a hard rain can spread the pathogen from the leaves and branches of a thick canopy to fruit as it is forming. Good aeration within as well as between trees can help prevent the fungus from spreading. Also, try to control insects like fruit flies that can cause injury to the fruit. Handle the fruit carefully when harvesting to limit injury and leave about an inch of stem in place to avoid stem end rot. Wash and dry fruit post harvest and keep them in a cool (not cold) place until ready to sell or eat.
Always, be sure to remove plant debris, leaves and broken branches from the area around the trees to avoid spreading any anthracnose spores. If you feel the problem is warrants treatment, you can spray a fungicide every two weeks from fruit set to harvest to will kill the disease. Copper compounds are effective as are other anthracnose specific fungicides.
More details on avocado anthracnose can be found in Scot Nelson’s UH CTAHR publication at https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-58.pdf.
If you want more information on growing avocadoes here in Hawaii, be sure to catch some of the speakers at the upcoming Avocado Festival this Saturday at Hale Halewai on Alii Drive in Kailua Village. Ty McDonald, from UH CTAHR, will do an avocado grafting demonstration at 10:30 a.m. and Ken Love, from Hawaii Tropical Fruit Growers, will discuss avocado varieties at 11:45 a.m. Either speaker could answer any of your avocado questions. Go to http://hawaiianwellness.com for more on the festival.
Email plant questions to konamg@ctahr.hawaii.edu for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant living in a dryland forest north of Kailua-Kona.
Gardening Events
Wednesday: “Little Fire Ant Control,” 5:30 to 8 p.m. at the UH Extension Office in Kainaliu. Hawaii Ant Lab will offer Instructions on controlling the Little Fire Ant (LFA). Vouchers will be available to reduce the cost of ant bait and spreaders for adults who complete the workshop Free. Registration is required. Info: http://www.hawaiicounty.gov/rd-agriculture. Register: http://bit.ly/2eYWIvx.
Saturday: “Work Day at Amy Greenwell Garden,” 9 a.m. to 12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Volunteers should bring a brown bag lunch. Water and snacks provided. Call Peter at 323-3318 for more information.
Farmer Direct Markets
Wednesday: “Sunset Farmers Market,” 2 p.m. to sunset at the north makai corner of the Kmart parking lot
Wednesday and Friday: “Hooulu Farmers Market,” 9 a.m. to 2 p.m. at Sheraton Kona Resort and Spa
Friday: “Pure Kona Market,” 9 a.m. to 5 p.m. at Amy Greenwell Garden in Captain Cook
Saturday: “Keauhou Farmers Market,” 8 a.m. to noon at Keauhou Shopping Center
“Kamuela Farmer’s Market,” 7 a.m. to noon at Pukalani Stables
Sunday: “Pure Kona Green Market.” 9 a.m. to 2 p.m. at Amy Greenwell Garden in Captain Cook
Tuesday through Saturday: “U-Pick greens and produce,” 10 a.m. to 4 p.m. Tropical Edibles Nursery, Captain Cook.
Plant Advice Lines
Anytime: konamg@ctahr.hawaii.edu
Tuesdays and Thursdays: 9 a.m. to noon at UH-CES in Kainaliu, 322-4892
Mondays, Tuesdays and Fridays: 9 a.m. to noon at UH CES at Komohana in Hilo, 981-5199 or himga@hawaii.edu