Scaffolds 01 index

(Dave Rosenberger, dar22@cornell.edu, Plant Pathology, Highland)

(Bill Turechek, wwt3@nysaes.cornell.edu & Cathy Heidenreich, mcm4@nysaes.cornell.edu, Plant Pathology, Geneva)

Introduction

Brown rot, caused by the fungus Monilinia fructicola, is a major disease of peaches, cherries, plums, prunes, nectarines, and apricots. The fungus can infect the blossoms, immature and mature fruit, spurs, and small branches. Complete crop loss can occur if weather conditions favor disease development and fungicide protection is lacking during bloom and just before ripening. Additional losses are possible in storage if fruit are not handled properly during harvest.

Symptoms on blossoms, twigs, and fruit

Disease symptoms are similar on all stone fruit. Infected flowers turn brown, wither, and either become fixed to twigs as a gummy mass or drop like unpollinated flowers. In order of susceptibility, apricot is most susceptible to blossom blight, followed by prune, sweet cherry, peach, sour cherry, and then plum. If infected blossoms do not drop off, the fungus may grow through the pedicel (flower stem) into the twig below, causing twig infections. Twigs develop elliptical to fusoid cankers with profuse gumming at the margin between diseased and healthy tissue. Leaves on infected shoots turn brown and wither, but remain attached. In some instances, twigs are girdled and killed.

On the fruit, brown rot infections first appear as soft brown spots. These rapidly expand and become covered with powdery masses of tan spores (called conidia). Infections may spread rapidly from fruit to fruit, particularly if environmental conditions are favorable and the fruit are touching one another. Under optimum conditions, an entire fruit may be rotted in 48 hrs! All stone fruits become increasingly susceptible to brown rot as they ripen. Rotted fruits typically shrink into a wrinkled "mummy" as they dry on the tree. Both immature and mature fruit infected with brown rot tend to remain on the tree and form mummies.

Disease cycle

M. fructicola overwinters in dried infected fruit called mummies, or in cankers on twigs and branches. Mummies remain either hanging in the trees or scattered on the orchard floor during the winter. Both may produce spores that infect blossoms and young fruit in the spring, but the mummies that remain in the trees are more important than those on the ground.

Two types of spores are produced from mummified fruit. Ascospores are produced only on mummies that have fallen to the ground and have been partially covered with soil. Ascospores are less common in the Northeast than in semi-arid climates. In the northeast, most brown rot infections develop from conidia that are produced on mummies and infected twigs. Conidia are produced in late spring when temperatures range from 55—77°F and are spread by wind, rain, and insects.

Warm, wet weather favors brown rot infection. Although conidia can germinate and infect at temperatures between 32 and 90°F, optimum temperature for blossom infection of peach range from 70 to 77°F. Under these conditions, spores germinate and penetrate plant cells on wet blossom surfaces in as little as 5 hours. In tart cherries, significant blossom infection can occur following 12 hr of wetness at 60°F or 24 hr of wetness at 50°F. Blossom blight may also develop at lower temperatures with prolonged wetting periods.

Disease Management

Orchard sanitation is essential for reducing disease pressure. Prune out mummified fruit and cankers during the dormant season and either burn or bury them deep in the soil. Remove wild or neglected stone fruit trees in the area that may serve as reservoirs for disease. Fruits thinned after pit hardening are more likely to become infected on the orchard floor than those thinned prior to pit hardening because they decompose less rapidly, but the importance of thinned fruit on brown rot epidemics in the northeast has not been studied.

Any type of injury will provide a point of entry for the fungus: hail damage, insect feeding wounds, bird pecks, fruit cracking, limb rubs, twig punctures, picking/packing injuries. It is essential to control fruit feeding insects such as plum curculio, oriental fruit moth, and tarnished plant bug. Take special care during harvest and packing not to puncture or bruise fruit. Cool fruit to as close to 32°F as possible after harvest.

Fungicide program

Some of the label information and restrictions for brown rot fungicides are summarized in Table 1. The protectant fungicides (Bravo, captan, sulfur) must be applied prior to a wetting period to be effective. If disease pressure is not very high, captan may be a good choice for blossom bight sprays because it is economical. Be aware, however, that captan can be phytotoxic to some sweet cherry and plum varieties. Bravo is a better choice for brown rot control on sour cherries and plums because it also controls black knot. Bravo is also the better choice when disease pressure is high, but it cannot be applied beyond shuck split.

The sterol-inhibiting (SI) fungicides include Elite, Indar, and Orbit. All of them are labeled for controlling blossom blight and can be applied again 2 to 3 weeks prior to harvest to control fruit rots. None of them are labeled for brown rot control at shuck split or first cover, but if applied at these times to control other diseases on the label (mildew, peach scab, cherry leaf spot, etc.) they will also suppress brown rot infections on green fruit. Labels for these products contain varying limitations concerning which stone fruit can be sprayed, spray timing, numbers of applications per season, etc., so read labels carefully. SI fungicides should not be used exclusively for both blossom blight and fruit rot; these fungicides must be rotated with non-SI fungicides for effective resistance management. The SI fungicides can provide 24—48 hrs of kickback activity if conditions prevented a timely application of a protectant fungicide prior to an infection period.

The benzimidazoles were once very effective brown rot fungicides. Widespread resistance to this class of fungicides has left them ineffective for most areas in New York State. The benzimidazoles may provide effective brown rot control in young orchards in isolated locations where resistant strains from older orchards are unlikely to be present. The benzimidazoles used in combinations with other brown rot fungicides can suppress black knot if applied at 7-day intervals between white bud and shuck split. Rovral is a dicarboximide fungicide labeled for use against blossom blight. It should be used as a protective spray, although it does have limited post-infection activity (~48 hrs at 68°F). Vangard is in a different class of fungicides and, like Rovral, is labeled for only blossom blight control. It is labeled for use on all stone fruits EXCEPT sweet cherry. Vangard has yet to be extensively tested for blossom blight in New York.

Final Considerations

For many stone fruits, only one blossom blight spray may be needed unless disease pressure is high. Where large numbers of fruit were left unharvested the year before, or when conditions are warm (above 60°F) and wet, more than one blossom blight application will be required. Petal fall applications are essential if bloom sprays were omitted and conditions turn warm and wet at petal fall. Fruit can be very susceptible to infection 1—3 weeks after shuck split, so shuck split and first cover sprays are important, especially in wet weather. Spray intervals should be tightened 3 weeks prior to harvest when fruit are most susceptible to brown rot. In order to manage disease resistance, SI fungicides such as Indar, Elite or Orbit should not be used continuously throughout the season for BOTH blossom blight AND fruit rot control. Use captan or other fungicides intermittently with preharvest SI fungicides. Lastly, ALWAYS remember to check product labels for timing and rates of application.

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(1) PB = pre-bloom (red bud for apricot, popcorn for cherry, pink for peach and nectarine, and white bud for plum and prune; PF = petal fall; SS = shuck split; Covers = cover sprays; PHI = pre-harvest interval.

(2) A = Apricot; C = Cherry; N = Peach and Nectarine; P = Plum and Prune.

(3) Do not apply Bravo after shuck split. On apricot, petal fall applications of captan should be made at 75% petal fall. Application of sulfur to mature nectarines may cause discoloration.

(4) Elite is also labeled on cherry for control of leaf spot and powdery mildew beginning at petal fall until terminal growth stops. On peaches, Indar can be applied for control of peach scab and on cherries for control of leaf spot beginning at shuck split at 10—14-day intervals up to harvest. Do not apply Orbit to 'Stanley type' prunes; do not apply to prunes after petal fall; do not apply more than 12 oz from early bloom through petal fall; no more than 2 applications are permitted for fruit rot control. Two additional applications can be applied to all stone fruits for control of powdery mildew or, on cherry, for leaf spot.

 

(5) If resistance is not an issue, these may be used in a fungicide resistance program. Fruit rot applications can begin 3 weeks prior to harvest. Topsin-M and Benlate should not be used alone.

(6) Do not make more than 2 applications per season.

(7) Do not apply to sweet cherries.