Fact Sheet FS1361
Anthracnose fruit rot is an economically important fungal disease that can affect berries, blossoms, inflorescences, and shoots of highbush blueberry (Vaccinium corymbosum). The disease can be expressed as a pre- or post-harvest fruit rot. Each year prior to harvest, 3 to 5% of blueberries are lost due to anthracnose, but this may increase during wetter growing seasons. Post-harvest losses can be as high as 100% due to asymptomatic yet infected fruit being harvested unknowingly. Blueberry anthracnose fruit rot can readily and rapidly spread amongst fruit in post-harvest conditions via asexually produced spores known as conidia. Losses at this stage can be devastating depending on the level of infection and storage conditions.
Causal Agent
Figure 1. Anthracnose infected fruit with sunken lesions, and in the processes of sporulation (conidia formation). Note the characteristic orange hue on the fruit surface. Neighboring berries may be infected yet remain asymptomatic.
The causal agent of blueberry anthracnose falls within the fungal genus Colletotrichum. Two species, C. gloeosporioides and C. acutatum, have been reclassified as species complexes and now, each one, represents several unique species. These species complexes cause fruit rots of many fruit crops, including apple, blueberry, cranberry, grape, and strawberry. Colletotrichum fioriniae (of the C. acutatum species complex) is the primary causal organism of anthracnose fruit rot in New Jersey, but other species from the C. gloeosporioides complex can also infect highbush blueberries.
Diagnosis
Berries are the primary tissue affected by anthracnose. Symptoms become more evident as the berries ripen, with sunken lesions, softening, and formation of viscous asexual conidia ranging from yellow to orange in color (Fig. 1). Although less common, symptoms may include blossom blight (Fig. 2a), twig blight, leaf spot, and cane cankers characterized by tissue necrosis. These symptoms can often be confused with other fungal diseases of highbush blueberry, including Phomopsis canker and twig blight and Botrytis blossom blight.
Figure 2. Conidia of the anthracnose fruit rot fungus forming on highbush blueberry. 2a Spores forming on a dying blossom with an inset showing the rice-grain-shaped spores taken at 400x magnification. Fig. 2b displays Colletotrichum sporulating on a blueberry bud.
Life Cycle
Figure 3. Location of bud scales in relation to the ovaries (developing fruit) showing the relative proximity of the reservoir of spores and the susceptible tissues.
The conidia of anthracnose are carried short distances by splash dispersal in water droplets from rain and overhead irrigation. As a result, disease outbreaks depend on the amount of inoculum (conidia or appressoria) surviving in the field from the previous season. The fungus can survive on woody tissues (e.g., canes, twigs) that remain in the field during the winter months. Infected inflorescence buds, which are set during the year prior, are the most significant source of primary infection (Fig. 2b). The outer bud scales can account for approximately 60% of the overwintering inoculum. Given the proximity of these bud scales to inflorescences and subsequent developing fruit, C. fioriniae can be easily spread (Fig 3).
The timing of infection is dependent on two factors. First, is the availability of inoculum that disseminates from infected bud scales, which are deposited on to the susceptible plant surface. Second, are the environmental conditions (e.g., moisture duration and air temperature) that promote the infection process. In the spring, the disseminated conidia can infect blossoms and green, unripe fruit. Infections begin when flowers open in April when temperatures are between 52 °F and 86 °F, with optimal conidial germination temperatures at 68 °F. The base of the flower contains the ovary that develops into the fruit after pollination. Blossom infections remain latent (asymptomatic) until the fruit ripen, leading to conidiation on the fruit and dispersal onto other tissues. During harvest, hand-picking, machine-picking, and storage of fruit may lead to further spread of the disease. Symptoms can develop one to two weeks into the storage period and can spread rapidly as fruits begin to breakdown (i.e., liquefy) and moisture development disperses conidia directly onto other fruit or processing equipment.
Management
Figure 4. Development of anthracnose fruit rot post-harvest. These fruit were removed from the packing line using electronic sorters. The orange-colored spores are easily transmitted to heathy fruit through either direct contact or indirectly by contaminating surfaces in the packing line.
There are three synergistic management approaches for blueberry anthracnose fruit rot. Sanitation and cultural measures, such as pruning out dead twigs, trusses, or canes on plants (in the fall and winter months), will reduce tissues in the field that harbor C. fioriniae. However, infected flower buds are typically not removed. Bushes should be pruned to improve airflow between canes and twigs, and should be spread apart for airflow between bushes, thus reducing moisture duration and relative humidity within the canopy. If overhead irrigation is the only option, it is best to water earlier in the day to reduce the amount of time aerial plant parts remain moist. This is known as the leaf wetness period, which if reduced, can have a profound impact on reducing infections. Drip irrigation will avoid aerial plant parts, again reducing the likelihood of increasing infection due to watering practices. During ripening (June–July), it is important to harvest fruit promptly as overripe fruit are more susceptible to anthracnose pathogens. Once fruit is harvested, refrigeration of fruit as soon as possible can greatly reduce the risk of postharvest anthracnose losses.
Cultivar selection and phenology of these cultivars are important considerations. Although no highbush blueberry cultivar is fully resistant to anthracnose, choosing less susceptible cultivars can reduce overall fruit losses. Commonly grown New Jersey blueberry crops include moderately resistant "Draper" and "Elliott;" slightly resistant "Duke;" and more susceptible "Bluecrop."
Bloom is the period where C. fioriniae will often begin to infect new, healthy tissue in the spring and is therefore considered a critical management window. Floral components present during bloom significantly stimulate repeating sporulation events (microcyclic or secondary conidiation), increasing the inoculum potential in addition to increasing the number and speed of infection structure (appressorium) formation. Considering this, preventative fungicide applications during bloom (April–May) are key in managing blueberry anthracnose. Refer to extension resources and fungicide labels for more information on application methods and local regulations.
References
February 2025
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