Monilinia oxycocci (Woronin) Honey, (synonym Sclerotinia oxycocci), common names cranberry cottonball, cranberry hard rot, tip blight, is a fungal infection of large cranberry (Vaccinium macrocarpon) and small cranberry (Vaccinium oxycoccos). The tips of young flowering shoots wilt before they flower. Fruit that forms on the plant can then be infected by the asexual spores traveling through the plant, causing the berries to harden, turn cottony on the inside, and dry out instead of maturing. The berries are filled with a cotton-like fungus and are generally yellowish with tan stripes or blotches at maturity, making them unmarketable. It results in important economic impacts on many cranberry marshes, particularly in Wisconsin.
Distribution
Cottonball occurs on cranberry marshes in the Pacific Northwest and southeastern Canada, but is more common in Wisconsin. The disease has become much more problematic in Wisconsin since the 1970s “for reasons that are not known.” Cottonball is considered the most important field rot in Wisconsin; other field rots such as early rot are rarely encountered. Typically 2 to 10% of fruit in diseased beds is infected; if left unchecked the amount can exceed 40%. Cottonball has been observed on all of the popular cranberry varieties in Wisconsin.
Signs and symptoms
When young cranberry shoots become infected, they show “tip blight” symptoms: shoot tips shrivel and a tan discoloration spreads from the tip of the shoot down the stem and into leaves. Cottonball tip blight differs from other shoot diebacks, in that tan V- or U-shaped lesions can be observed centered on the leaf midvein. Also, white, powdery conidia (asexual spores) appear on diseased shoots just before and during bloom. Tip blight of cottonball is often inconspicuous and is easily overlooked.
Immature diseased berries show no external symptoms, but are filled with white, cotton-like fungus. While healthy berries turn red as they mature, diseased berries may acquire a red blush but are generally yellowish with tan stripes or botches. Diseased fruit are unfit for fresh or processing markets.
Dried-up remains of diseased fruit contain sclerotia and are commonly called “mummies.” The “mummies” are roughly spherical and about ½ inch in diameter. In the spring, tan or brown mushroom-like structures (apothecia) about 1–2 inches tall grow from the mummies. “'Mummies' and apothecia are difficult to spot."
Disease cycle
Monilinia species attack various Ericaceae hosts. M. oxycocci on cranberry exhibits a disease cycle similar to other species and is characterized by two distinct phases: infection of shoots (tip blight), and secondary infection of flowers (leading to fruit rot). Fruit rot only results from the infection of flowers during bloom.
About the time of budbreak in the spring, apothecia develop from overwintered pseudosclerotia—the hardened, dried-out “mummified” remains of previously diseased fruit. Mummies are typically spherical and approximately 0.5 inches in diameter. Apothecia are generally 1 to 2 inches tall. These apothecia discharge ascospores over 10–14 days. Ascospores infect succulent young developing shoots (0.5 – 1.5 inches long), causing tip blight. This is the primary infection. Come bloom (about 3–4 weeks after primary infection), conidia are produced on the infected shoots and are dispersed to infect open flowers. Conidia borne either by wind or insect deposition may land on the flower stigma, germinate, grow down the style, and colonize the ovary. This is the secondary infection.
Within the ovary, fungal mycelia form a white cottony mass in each of the four locules, or fruit seed cavities, and grow into the fleshy fruit tissue. The infected berry itself remains firm, categorizing this type of rot as a hard rot (a soft rot is characterized by total tissue maceration and seepage). Affected berries remain yellowish-tan and in some cases turning brown, instead of coloring normally. Eventually, the fungus consumes the fruit pericarp and a hard black pseudosclerotium (mummy) develops from 25 – 50% of the diseased fruit. Mature pseudosclerotia often float and may be dispersed by harvest or cold protection floods. The fungus will overwinter in these mummies—which can be difficult to detect—at or near the soil surface. Pseudosclerotia remain viable for several years. Wet conditions in the spring stimulate apothecia growth and the cycle moves forward. Berries without pseudosclerotia decompose by the next spring.
