WSU Whatcom County Extension

Integrated Pest Management for Raspberries

Botrytis Cane & Fruit Rot

(Botrytis cinerea)

Insects and Invertebrates

 

Botrytis

 

Symptoms

Botrytis fruit rot is found on overripe and bruised fruit and is more prevalent in fields under overhead irrigation or where fruit is allowed to become ripe enough to be harvested mechanically. Most infections occur on the flower during bloom; however, symptoms usually go unnoticed until harvest. Infected flowers turn brown and shrivel when dry conditions exist. Under moist conditions, grey tufts of fungus can be seen on blighted blossoms. Green fruit infections can be seen as purplish-grey hard berries.  Mature fruit infections usually appear as soft, light brown, rapidly enlarging areas. Berries become shriveled and covered with the grey tufts when the fruit matures. One to several berries in a cluster may show blasting (browning and dying) that may extend down the pedicle. The fruit may appear healthy at harvest but become severely rotten within a short time when conditions are favorable for disease development. A grayish, dusty, or powder growth of the fungus may develop, which is why it is commonly referred to as gray mold.

Pale brown lesions may appear on primocane leaves in mid to late summer. Cane infections first appear as brown lesions on new green canes, often encompassing more than one node. The lesion becomes tan as the infected cane matures. Cane lesions exhibit typical concentric “watermark” patterns from fall through late winter. Botrytis cane lesions may be confused with spur blight: however, lesions typically associated with spur blight are much darker in color. Sclerotia may be visible on canes as shiny, black, blister like structures. Cane infections can be very destructive during wet season and in plantings where the growth is lush and dense. Cane lesions can girdle primocanes and fruiting laterals in some cases.

 

Life History

The pathogen, Botrytis cinerea, is the fungus that causes blossom blight, fruit rot and cane botrytis. It overwinters as minute, black, fungal bodies (sclerotia) on bleached appearing canes or as mycelium in dead leaves and mummified berries. In early spring, under humid conditions, sclerotia produce conidia (spores), which are dispersed by wind, rain, and overhead irrigation. Preferable conditions for disease development are high humidity and temperatures between 70 and 80˚F. Disease will develop under cooler temperatures, but only if foliage remains wet for a longer period. Spores can infect mature or senescent leaves, resulting in primocane infections through petioles. Infected new canes wilt, die and may be covered with grey mold. During bloom, the fungus colonizes healthy or senescing flower parts turning blossoms brown. The fungus is then able to establish within the receptacle of the young fruit as a latent infection. Infections generally remain dormant until fruit is nearly ripe or after harvest. Infections can recur throughout the season by sporulation of the fungus on unpicked, leaky, overripe fruit left on the vine.

 

Botrytis

 

Monitoring

Scout 3-5 sites per field, depending on field size, and evaluate 10-20 hills spaced 3-5 hills apart at each site. Record severity of leaf, cane, and fruit symptoms on a scale of 0-3.

Watch for infections in the spring, which appear as bleached-out, whitish areas on the cane. These overwintering infections are a primary source of spores for flower infections, which may develop into fruit rot symptoms. Monitor leaves and canes in early summer for new infections, which first appear as brown lesions on new green canes. Green fruit can be monitored for purplish-grey hard berries. Monitoring is particularly important when wet conditions occur during bloom and harvest and/or in fields containing heavy foliage. During harvest, check fruiting laterals for infected berries paying particular attention to over-ripe berries which may be showing beginning stages of infection.

 

Botrytis

 

Thresholds and Management

No threshold or tolerance level available.

Examine canes in the spring to determine the level of overwintering fungus and plan a control program accordingly. If chemical treatment is required, a protectant fungicide is usually necessary in the early bloom stage (10% bloom) in established, bearing fields. This is usually followed within 14 days with a second protectant fungicide application. Additional treatments may be necessary prior to harvest, but this will depend on weather conditions, incidence of fruit infection, and method of irrigation. The frequency of fungicide applications is greatest in fields destined for high-end markets, which have a very low tolerance for diseased fruit. Be sure to alternate between fungicide classes to avoid resistance problems.

Promote air circulation and proper drying of plant tissue through pruning and trellising to open plant canopy. Maintain a narrow row by burning back early first year primocanes and controlling weeds. Minimize or adjust irrigation to prevent plants from being wet for extended periods of time. Avoid excessive use of nitrogen fertilizer in the spring because the Botrytis fungus will readily infect succulent green growth. Pick fruit before reaching full maturity to prevent postharvest fruit rot. Pick fruit often and early in the day when temperatures are cool. Move harvested fruit to cold storage as soon as possible. Berries infected by Botrytis are usually not shaken off the plant by the harvester; hence you will not get an estimation of disease severity by inspecting berries as they come across the belt. If chemical treatment is required, fungicides can be applied as a protectant spray at 7-14 day intervals from early bloom up to harvest.

 

Resources

Ohio State University Extension, Botrytis Fruit Rot “Gray Mold” of Strawberry, Raspberry, and Blackberry
http://ohioline.osu.edu/hyg-fact/3000/pdf/HYG_3017_08.pdf

Oregon State University Extension, Plant Disease Control: Raspberry – Fruit Rot and Cane Botrytis
http://plant-disease.ippc.orst.edu/ShowDisease.aspx?RecordID=947

University of California, IPM Online: Caneberries, Botrytis Fruit Rot
http://www.ipm.ucdavis.edu/PMG/r71100211.html

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WSU Whatcom County Extension 1000 N. Forest St., Suite 201, Bellingham, WA 98225 (360) 778-5800 whatcom@wsu.edu