I. Introduction: Brown rot is one of the most important diseases of stone fruits in the mid-Atlantic region. Field losses of nectarines can be extensive if conditions favorable for disease development occur during the blossoming or preharvest and harvest periods. Losses of peach vary with susceptibility of the cultivar.

II. Symptoms: Typical disease symptoms induced by M. fructicola include blossom and twig blight (photo 2-49), cankers (photo 2-50), and a fruit rot (photo 2-51). The fungus often produces conidia profusely on sporodochia on infected areas. The first indication of the disease in the spring is the rapid death of blossoms which, as they turn brown, often become affixed to the twig in a gummy mass, later becoming covered with a grayish to tan spore mass. Frequently, following colonization of the blossom, the fungus enters the shoot where it causes a canker on which spores are also produced. Shoot blight symptoms will occur if the fungus girdles the shoot. Leaves on such shoots turn tan to brown and may remain attached for several weeks. Cankers formed following blossom or fruit infection appear as brownish, sunken areas, that are often covered with gum. These cankers support the formation of conidia in wet weather and harbor the fungus over the winter. Usually, the tree is able to restrict cankers to small oval areas at the junction of the shoot and the infected blossom or fruit. Cankers and killed shoots may be colonized by other aggressive canker-causing fungi such as Leucostoma spp.

Brown rot on ripening or mature fruit typically develops as a rapidly spreading brown necrosis. Under optimum conditions for the fungus, entire fruit may be rotted within 48 hours of infection. The infection produces a soft dry rot, although occasionally the skin remains firm. On nectarines, brown rot sometimes occurs as quiescent infections which can be detected as small, circular, necrotic lesions on immature fruits. As fruits mature, decay spreads from the lesion throughout the entire fruit. Immature or mature fruit with brown rot infections will sporulate profusely, shrivel, and become tough grayish-black mummies (photo 2-52). These mummies may drop to the ground, where apothecia (photo 2-53) may develop, or remain attached to the tree through the winter. Decaying fruit in cold storage or transit may appear black with little or no sporulation.

III. Disease Cycle: M. fructicola overwinters in orchards as mycelium on mummies, fruit stems, blighted blossoms and twigs, and cankers. Sporodochia develop under cool, wet conditions during the winter and early spring. Occasionally, cup-like apothecia of M. fructicola which produce ascospores can be found on fruit mummies under the tree, but they are not usually common in mid-Atlantic commercial orchards. In years when apothecia were common, severe blossom blight was noted in peach and apricot orchards, but severe blossom blight also can occur in the absence of apothecia. Generally, conidia from mummies and cankers on stone fruit trees and other sources (for example, flowering ornamental plants of plum or quince, or wild plantings of plum) are believed to be the primary inoculum sources.

Conidia of M. fructicola are generally formed during late spring when temperatures range from 55 to 77 F (13-25 C). Conidia are disseminated by wind and rain and germinate rapidly under favorable conditions. Optimum temperatures for blossom infection of peach range from 72 to 77 F (22-25 C). Between 32 to 86 F (0-30 C), temperatures above or below the optimum range delay germination but do not inhibit it. Inoculum concentration also interacts with temperature and wetness duration to influence incubation period and disease incidence and severity. Under optimum temperature conditions, fruit infections can occur with only three hours of wetness when inoculum levels are high. Longer wet periods during infection result in shorter incubation periods so symptoms develop more rapidly. Large amounts of inoculum with highly favorable environment produces a high potential for heavy losses.

Although blossom blight can be severe enough to reduce the crop, early sporulation on even a few infected blossoms provides more inoculum for later fruit infections. The subsequent invasion of shoots also enables the pathogen to survive in the host for long periods. In some areas, infections of flowers may result in active or quiescent infections that either cause decay of green fruit or become active prior to harvest. Quiescent infections of peach and nectarine have not been reported in the eastern U.S., perhaps because blossom infection is less common here than in other locations.

Sporodochia of M. fructicola on infected blossoms and shoots may produce viable conidia throughout the remainder of the growing season, although sporulation from infected blossoms tends to decline over the summer. These conidia may infect injured green fruits; nonabscised, aborted fruits; green fruits thinned after the pit hardening stage and dropped under the trees; and ripening fruits as they mature. For most stone fruits, susceptibility of fruit to infection increases as fruit color begins to develop. Infection may occur by direct penetration of the germinating spore through the cuticle or lenticels in the fruit. In most orchard situations, inoculum produced on early maturing cultivars fuels a continuing outbreak that affects late maturing cultivars.

Insects (nitidulid beetles and honey bees) also can be important as vectors of the fungus during fruit ripening, carrying conidia to injury sites produced by oriental fruit moth, Japanese beetle, green June beetle, stink bugs and other insects that injure fruit. Wounded fruit are infected much more readily than nonwounded fruit. At harvest, apparently healthy fruit usually are contaminated with spores which, under favorable conditions, may later decay during storage and marketing.

IV. Monitoring: During or after pruning (before the pink stage), monitor a minimum of 20 sample trees per block for the presence of fruit mummies (photo 2-52) and cankers (photo 2-50). A total of one to ten mummies and/or cankers, and more than ten mummies and/or cankers represents levels of moderate and high risk, respectively, for blossom infection under the appropriate environmental conditions.

Before bloom, monitor the orchard floor under sample trees for the presence of apothecia of the brown rot fungus. These are more likely to occur in the wettest areas of the orchard on mummies partially buried in soil and/or among weeds (photo 2-53). Finding any apothecia represents a potential high risk for blossom infection. Remove cankers (photos 2-49, 2-50) surgically if possible or prune out the entire diseased area. Monitoring for and removal of cankers is best done at the same time.

At shuck fall, examine ten shoots on each sample tree for the presence of blossom infection (photo 2-49). A total of one to ten blossom infections and greater than ten blossom infections represents moderate and high risk, respectively, for fruit infection during the preharvest and harvest periods.

Fruit susceptibility to brown rot (photo 2-51) increases rapidly as fruit begin to color. Monitor ten fruit on each sample tree for disease incidence. Greater than two infected fruit per ten acres (eight trees sampled) represents a high risk for a brown rot outbreak at this time. Monitor approximately every three to five days during the preharvest period. Insect, bird and hail damage to ripening fruit can result in wounds which can be quickly colonized by the rot fungus. Brown rot will show up first in areas near sources of inoculum and where fruit may be physically injured. An estimate of the potential for postharvest rot to develop can be determined by harvesting 10 mature, healthy fruit from each sample tree. Wound each of these fruits several times by stabbing them with a knife [1/4 inch (6-7 mm) deep] and holding them in a closed plastic bag at room temperature for 24 to 48 hours. Record the percent rotted fruit.

Brown rot may develop during storage and shipment if fruit are not handled properly during and after harvest. Monitor daily for developing decay in packed fruit being temporarily stored by checking fruit throughout a minimum of containers.

V. Disease Management

Cultural practices. Sanitation is essential if your orchard is to be considered a low risk for a brown rot epidemic. The practices listed below, if followed, should minimize brown rot spore populations and limit the likelihood of an epidemic when conditions are favorable for rapid disease development.

(1) Remove all remaining fruit from the tree after the final picking. This practice limits infection of fruit peduncles and twigs thus reducing the number of brown rot cankers. In addition, this practice prevents the situation where overwintered mummies within the tree would be immediately adjacent to susceptible blossoms in the spring. Furthermore, removal of remaining fruit after final picking separates the practice of removing mummies from spring pruning. Where these practices are separated, the grower has more latitude to selectively prune (following a severe winter with high bud mortality, for example) without increased risk of blossom infection.

(2) Fruit thinning practices influence the carry over of brown rot during the summer months and into the fruit ripening season. In general, fruit thinned before pit hardening decompose rapidly; whereas, fruit thinned after pit hardening become infected on the orchard floor and serve as spore sources for the disease. Although it would be ideal to thin all cultivars before pit hardening, this is not practical because: (i) early, mid-season and late cultivars reach the pit hardening stage at the same time; and (ii) thinning early season cultivars at pit hardening or before favors formation of split pits. If thinning is done after pit hardening, the thinned fruit should be removed from the orchard (thin with picking baskets or rake and shovel the litter). Alternatively, direct a sprayer nozzle downward so that thinned fruit receive some fungicide.

(3) In spring, monitor for blossom infection and prune out any cankers and infected shoots.

(4) In spring, just prior to and during the blossom period, examine the orchard floor for apothecia. Their presence requires that blossoms be thoroughly protected with fungicide sprays during wet periods.

(5) Prune to avoid excessive overcrowding of branches to increase air circulation, promote rapid drying, and increase light and spray penetration.

(6) Fertilize to maintain optimum nitrogen/potassium balance.

(7) Avoid dumping rotten fruit in one location, which could become the starting point for disease and insect outbreaks in the following season.

Chemical management: Fungicides are recommended generally in a protective program for a complex of diseases, including brown rot, scab, and powdery mildew. Fungicides are to be applied prior to fungal infection that occurs during rain periods. Here is a table describing the relative effectiveness of peach fungicides.

Rating scale: E = excellent, generally good disease control under heavy disease pressure; G = good, good control under moderate disease pressure; F = fair, fair control under moderate disease pressure; S = slight, some control under light disease pressure; N = little or no effect on indicated disease; - = information lacking or not applicable.

Caution: Combinations involving Benlate or Topsin-M may become ineffective for scab or brown rot control if resistance to these fungicides develops. If resistance is suspected, switch to a fungicide program not involving Benlate or Topsin-M until the fungus has been tested for sensitivity.

Credits: Text prepared by A. R. Biggs, modified from the original text in the Mid-Atlantic Orchard Monitoring Guide (original text by A. R. Biggs, K. D. Hickey, and K. S. Yoder). Table of fungicide effectiveness from the 1997 Va./W.Va./Md. Spray Bulletin for Commercial Fruit Growers, table compiled by K. S. Yoder and A. R. Biggs.