Blueberry Anthracnose
(Ripe Rot)

Anthracnose or ripe rot is caused by the fungi Colletotrichum gloeosporioides and C. acutatum. Recent surveys have shown that anthracnose occurs in all blueberry producing areas in B.C., but not in all fields. The primary concern with anthracnose fruit rot is reduced shelf life and poor quality fruit for the fresh market.

Symptoms

The fruit is the most common part of the blueberry that is infected. Berries do not develop symptoms until they are mature or almost mature, Figure 1. The initial symptoms of infection are shriveled, sunken areas. Within a few days, bright orange spore masses are produced within the shrunken areas, Figure 2. The sticky spore masses spread to other fruit and twigs by splashing water and contact during harvest.

Spent fruit trusses appear to be the main site where the fungus overwinters. When trusses are infected during the growing season they do not show obvious symptoms. However, in the spring it is possible to observe orange spore masses produced on infected trusses, Figure 3.

New shoots and leaves may also become blighted by the anthracnose fungus in the spring. This symptom has not been observed in B.C.

Figure 1.
Fruit infected with anthracnose

Figure 2.
Orange masses of spores oozing out of a berry

Figure 3.
Orange spore masses of anthracnose on a spent fruit truss

Conditions for Infection

Colletotrichum gloeosporioides requires a minimum of 12 hours of continuous leaf/berry wetness when temperatures are between 15 to 27°C for infection. The conditions needed for Colletotrichum acutatum are not known, but temperature requirements may be less than 15°C. Studies by BCMAFF and in Washington State by Dr. Peter Bristow have shown that infection can occur throughout the period of berry formation and development.

Although berry infections are only obvious when fruit is mature, the fruit tissue may be infected anytime from flowering to harvest. After the fungus enters the developing fruit it remains dormant until the berry starts to ripen. At this point, the fungus begins to grow and produce enzymes which destroy plant cells. Damaged cells lose their cell contents and deflate like a punctured balloon. When enough cells in one area are attacked this way, the entire area becomes shriveled.

In B.C., it takes approximately 4 ½ months from flowering to harvest. Infection could occur anytime, and it is this lengthy period of susceptibility that creates the challenge in managing anthracnose fruit rot.

Disease Prevention

Rainy weather and overhead irrigation during fruit development provides the environment for infection. There is nothing that one can do to avoid wet weather. However, practices to speed the drying off of plants will help.

• Prune to improve air movement where bushes have dense foliage and are planted close together. Opening the canopy encourages faster drying of leaves and fruit tissue. Although the anthracnose fungus overwinters on infected twigs and trusses, it is not feasible to attempt removal of all dead twigs and trusses. Trials in Washington State found that pruning every dead twig and truss from bushes only reduced infections by 50%. The high cost associated with this type of pruning makes it uneconomical.
• Where overhead irrigation is needed, it should be started early in the morning so that plants can dry off during the day. The objective is to keep the plant wetness period under 12 hours.

Spores may also be spread around on flats, totes and machine harvesters.

• Make every effort to clean equipment that is moving from infested to uninfested fields.

Disease Management

Where anthracnose has been a problem, apply Bravo at petal fall. Use at 7.2 L/ha (2.9 L/acre). Apply in 1000 L/ha (400 L/acre) of water. Do not apply within 54 days before harvest. After petal fall, protectant fungicides such as captan applied for Botrytis will continue to provide some protection. In unusually mild springs, apply Bravo at green tip and pink bud also.

An emergency registration of Quadris was obtained for the 2002 season. For fields with anthracnose, the label reads as follows:

Quadris at 1.0 L/ha (0.4 L/acre) in enough water to obtain thorough coverage. Apply beginning at mid-bloom and continue through early fruit development. Sprays should be timed to provide protection when conditions are wet and average temperatures are over 15°C. Do not apply more than 3 times per year. Do not tank-mix with any other pesticides. Do not apply within 30 days of harvest.

The registration is valid until August 31, 2002. Some IMPORTANT additional points:

1. The preharvest interval (PHI) is 30 days. If this is not respected, fruit residues may exceed 0.1 ppm azoxystrobin and your crop may be in violation of the Food and Drugs Act.
2. Azoxystrobin is toxic to fish. Please take care using the product around water bodies when applying it and when cleaning equipment.
3. Quadris has been shown to be very phytotoxic to certain varieties of apples and crabapples. Do not apply where there is the possibility of spray drift reaching apple trees. Do not spray apples with sprayers used to apply Quadris.
4. Timing: It appears that infection occurs at temperatures near 12°C or more, and when conditions are wet for prolonged periods. Begin protection of blueberries when these conditions promote the disease.
5. Quadris does not provide protection against Botrytis. Therefore, if you have a Botrytis problem in your field, you will need to apply Bravo, captan/Maestro or ferbam to protect the blossoms and developing fruit. We have not seen any data to show that ferbam controls anthracnose. Bravo can be applied up to petal fall. In cooperating trials with Victoria Brooks, PARC Agassiz, last year, both of the following treatments reduced overall fruit rot infections by half. You could choose either:
   • Quadris, then Bravo, then Quadris (note Bravo cannot be applied within 54 days of harvest), or
   • Quadris, then captan/Maestro, then captan/Maestro, then Quadris.
6. Do not apply Quadris more than 2 times in a row. Alternating Quadris sprays with captan/Maestro or Bravo applications will help prevent the development of fungicide resistance.

Post-Harvest Fruit Handling

• Bring flats into the packing shed as quickly as possible after picking. Store filled flats in the shade if they cannot be transported right away.
• Remove field heat from fruit as soon as possible. Where blueberries are cooled to 2°C within 2 hours of harvest, there is less rot than if fruit cools to this temperature over 48 hours.

Photos 1 and 2 courtesy of Dr. P. Bristow, Washington State University, Puyallup, WA.