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Bacterial canker in Washington sweet cherries

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Written by Bernardita Sallato C., Gary Grove and Alexandra Johnson, Washington State University, August 2021. This is a DRAFT will be available at https://pubs.extension.wsu.edu/ when it is formally published.

 

Bacterial canker in sweet cherries is caused by the bacterium Pseudomonas syringae pv. syringae (PSS) van Hall (Romoli, 1963, Latorre et al., 1980), and is commonly associated with production areas characterized by wet and cold weather such as western Washington, Oregon, southern Chile, among others (Latorre, 2008). In the arid sweet cherry producing region of eastern Washington, bacterial canker was long considered of secondary importance. However, in recent years, the disease has been observed more frequently, especially in newer plantings. Tree losses of up to 75% have previously been reported (Cameron, 1970), with up to 60% incidence (proportion of trees affected) in 2021, in young orchards in WA. The apparent increase in disease levels observed in WA can be attributed to several factors, with the seemingly most evident being the high susceptibility of the cultivar ‘Coral Champagne’. Released in 1984, ‘Coral Champagne’ has been gaining popularity over the last decade in WA and was the third-leading cultivar ordered in 2012. Additional factors influencing bacterial canker infection include extreme weather conditions and irrigation practices. For example, in 2021, a frost event with temperatures below freezing impacted many tree fruit orchards in eastern WA during the month of April, followed by cool and wet conditions, ideal for P. syringae growth. Record high temperatures in June that continued throughout the summer lead to additional plant stress and increased susceptibility to infection. The presence of other diseases, such as Little Cherry Disease (LCD) or Verticillium Wilt can further predispose trees to infection by PSS. This article responds to the needs of many WA growers for additional information on bacterial canker caused by PSS and provides an overview of the disease, management practices for WA sweet cherry growers, and guidelines to better diagnose the disease and prevent confusion with other threatening diseases affecting WA sweet cherry industry.

PATHOGEN

Bacterial canker disease in sweet cherry is caused by the bacterium Pseudomona syringae pv. syringae and can infect numerous species including tart cherry, peach, plum, berries, and many weeds found on orchard floors. Additionally, P. syringae can form ice inside the plant cell and generate damage, called ice nucleation activity (INA; Lindow et al., 1978). Pathogen colonization can occur on any plant tissue without provoking infection or symptoms (Kennelly et al. 2007). Infection requires a wound or a natural opening accompanied by wet conditions or standing water. In infected trees, PSS survives in cankers and buds, which are the primary sources of inoculum for initial infections in the following growing season. PSS populations increase during spring after bud break and symptoms of infection develop over the spring and summer. In late summer, PSS populations decline; however, there can be a second spike of infection during the cooler, wetter conditions of autumn.

PSS can be transmitted by grafting and can translocate systemically throughout the plant. Vigorous rootstocks have a greater chance of root grafting and transmitting PSS to surrounding trees than dwarfing and semi-dwarfing rootstocks. The pathogen can also be dispersed and disseminated by wind, rain or irrigation water, and insects. PSS bacteria can be washed and transported by water droplets. Mechanical movement throughout pruning appears not to be of risk, but should not be overlooked if the equipment is in direct contact with fresh gummosis and conditions are favorable (i.e., cool and wet).

 

PREDISPOSING CONDITIONS

PPS requires a wound or other opening to enter the plant and initiate infection, thus, any condition that damage the tree, will increase the risk of infection. Bacterial canker is highly associated to winter freezes and spring frost events. Trees damaged by extreme weather conditions will have wounds that, when exposed to water (rain or irrigation) and cool temperatures (between 59 and 77 F (15 – 25 C)), are ideal conditions for infection to occur. Thus, PSS infection is most likely during the winter, spring or fall, however symptoms are commonly observed between spring and summer. In WA, most infections have been associated with heading cuts (during planting in the spring) that become exposed to irrigation water and cold temperatures. Freeze or heat damage in the trunk, pest wounds (e.g., deer and rodents), or mechanical wounds create favorable infection sites. Fresh leaf scars due to abscission in the fall, or by hand removal in the base of the new planted trees can also increase infection risk.

Weaker trees are also more susceptible to PSS infection, especially when trees are affected by other pathogens (e.g. viruses, phytoplasmas, Verticillium sp., Leucostoma, nematodes, etc.). Verticillium sp. have been found in combination with P. syringae in WA orchards. Environmental conditions that lead to additional abiotic stress, when the pathogen is present will increase the level of infection and promote symptom development, e.g. limiting soil conditions with alkaline soils, calcareous layers in the soil that limit root growth, nutrient deficiencies, water stress or extreme heat.

GENETIC SUSCEPTIBILITY

While all sweet cherry cultivars and rootstocks can get infected by PSS, some cultivars have been shown to be more susceptible than others.  In WA, the cultivars Sweetheart, Bing, Staccato, Royal Ann (Napoleon), and Van are susceptible while Coral Champagne has been shown to be highly susceptible. Higher levels of resistance have been reported in Corum, Regina, Moreau, Lambert, and Sam (Junior 2002, Spotts et al., 2010). Rainier has contradicting results but might be somewhat resistant in certain environments (Spotts et al., 2010, Roche, 2001, Mgbechi-Ezeri et al., 2017, Johnson et al. manuscript in preparation). Among the rootstocks, Mazzard and Colt seems to be more resistant, while more dwarfing rootstocks are more susceptible, e.g. Gisela 6 and Krimsk 5 (Spotts et al. 2010).

 

SYMPTOMS IN SWEET CHERRY

The most distinct symptoms of infection in sweet cherry is a dark canker sometimes accompanied by gummosis (reddish-brown exudate) (Latorre, 2008, CABI, 2020; Figure 1). New limbs can die back, affecting buds or newly developed blooms that remain attached to the spur (Figure 2 – 3) (Moore, 1988, CABI, 2020). Infection is more common in young trees, although it can occur in trees at any stage of development if conditions are favorable. Reddish, malodorous lesions in the root cortex are indicative of infection by PSS (CABI, 2020). Symptoms commonly develop at the base of trees, an area that, in young trees, receives higher exposure to damage by machinery, rodents, frost, and irrigation, providing numerous opportunities for PSS infection to occur. Note that there are other conditions that can lead to gummosis in sweet cherry. Generally, noninfectious gummosis is clear to amber color, thus diagnosis usually requires expert assessment or laboratory testing.

 

Figure 1.a Gummosis due to bacterial canker in a broken branch of sweet cherry (photo: B. Sallato).

Dark brown gummosis in cherry bark

Figure 1.b Gummosis due to bacterial canker in a broken branch of sweet cherry (photo: B. Sallato).

dry out cherry apical shoot

Figure 2. Limb with canker and dead flower buds that remain attached to the limb. (photo: B. Sallato)

wilted shoot

Figure 3. Limb dieback in sweet cherry ‘Coral Champagne’ on Gisella 6. (photo: B. Sallato)

When PSS infects the trunk or main leaders, it can create a girdle (Figure 4), weakening the whole tree or branches distal to the canker and inducing symptoms that are similar to water stress or nutrient deficiency (e.g., chlorosis, small leaves, small fruit, and tree death; Figure 5). Thus, PSS infection can be confused with other abiotic and biotic (e.g. Leucostoma canker) conditions that have the same symptoms. Additional symptoms of PSS infection are necrotic tissue beneath  the bark, with a defined reddish-brown pattern that affects the vascular system. This necrosis can be observed by cutting the bark and wood tangentially with a clean, sharp knife.

arrows pointing to clear gummosis in bark

Figure 4. Canker and callus developed in the lower part of the tree creating a girdle. Gummosis are present below the canker and include a weak limb with wilted leaves.

wilted tree

Figure 5. Sweet cherry tree infected by P. syringae generating a girdle in the main leader and weaking the tree. Symptoms can be confused with water stress or advanced decline due to little cherry disease, other viruses, and Leucostoma Canker.

Weak trees or limbs that have been girdled as a result of bacterial canker infection do not always appear with gummosis or signs of infection in the upper portion of the trees. Instead, these weaker limbs will have reduced water and nutrient uptake, leading to small fruit and yellowing leaves. Due to the high incidence of Little Cherry Disease (LCD) in Washington cherry orchards, caused by little cherry virus 1 (LChV1) and/or little cherry virus 2 (LChV2), as well as X-disease caused by Candidatus Phytoplasma pruni, symptoms can be confusing. Symptoms of LCD and X-disease are distinct when fruit is present, with mature, ripe fruit of adequate size present on the same limb or spur as unripe, small, and yellow fruit (Figure 6). Small fruit associated with bacterial canker infection are uniformly similar in size and color throughout the entire affected limb or tree (Figure 7). Additional symptoms such as cankers, gummosis, scars in the trunk, or overall tree decline can help distinguish between the diseases. Ultimately, testing for LCD, X-disease, or bacterial canker should clearly distinguish between pathogens. Bacterial canker is also sometimes confused with Leucostoma Canker (also known as Cytospora Canker) which is caused by the fungus Leucostoma cinctum. Cankers produced by L. cinctum tend to have a well-defined margin.  The canker surface can also contain minute, pimple-like structures known as pycnidia; the spores that spread the disease in orchards are produced in these structures.  Note that it is also possible to have multiple diseases in one tree.

smalle red cherries next to big dark red cherries

Figure 6. Little Cherry Disease (LCD) symptoms in sweet cherry ‘Skeena’ on Gisella 12. Ripe, dark colored fruit in an asymptomatic limb (right) shown next to light red to yellow small fruit in symptomatic spurs (left). Leaves are green, adequately sized, and with no disease symptoms.

tree with small fruit and leaves

Figure 7. Small fruit and overcropped ‘Early Robin’ tree affected by bacterial canker. The whole tree is weak, overcropped, with small size fruit above the girdle created by P.syringae.

CASES IN WASHINGTON SWEET CHERRY

‘Coral Champagne’ on mazzard rootstock orchard planted in 2021 in the Yakima valley. Trees were planted in April and headed back to approximately knee high immediately after planting to generate a ‘Y trellis’ formation. Irrigation is applied via micro-sprinkler. After planting and heading, water from the irrigation was in direct contact with the heading wound. Temperatures during tree establishment ranged between 30 and 50 F. A neighboring site planted with ‘Black Pearl’ on Gisella 12, showed no signs of infection. While it is not possible to determine if the new plants obtained from the nursery already had PSS or if the bacteria infected the trees post-planting, several conditions were conducive for the infection: susceptible cultivar, cool temperatures during spring, heading cut during spring, and irrigation in contact with the heading cut.

‘Coral Champagne’ on Krymsk 5 rootstock orchard planted in 2018. The site faces south, with a slope above 3 percent, and has a cool area in the southeast corner. The cooler area coincides with a soil section that has low drainage and high-water table. At planting, trees were headed back during spring and trained into a “Y trellis” system. Irrigation was provided mostly via drip; however, sprinkler irrigation was also used for frost protection and ground cover maintenance. Tree decline started after the first year of planting and progressed over subsequent years. In 2021, symptoms of bacterial canker infection increased dramatically after a frost event in April, which included three days of temperatures below freezing, was then followed by record high temperatures during the summer. Symptoms of infection found in 2021 included cankers, gummosis in buds and trunk, dead buds and blooms, small, yellow fruit, as well as whole tree decline due to trunk girdling. Tree condition improved towards the upper side of the block, where water in the soil had better drainage and temperatures were milder. The pollinizer ‘Chelan’, which was also present in the block, as well as the contiguous planting of ‘Rainier’ were found to have significantly fewer symptoms of infection.

In both cases involving the cultivar Coral Champagne, a highly susceptible scion was used and environmental conditions present were conducive to infection (cold and wet spring). Furthermore, in both cases, trees were headed after planting to generate a “Y trellis” formation. However, in the first case, infection-favoring environmental conditions were exacerbated by performing tree heading cuts during cool weather followed by wetting the cuts with micro-sprinkler irrigation. In the second case, PSS was present in the block and freezing conditions during spring could have damaged limbs and buds creating wounds. While the block has drip irrigation, micro-sprinklers utilized for frost protection and lack of soil drainage provided the wet conditions needed for PSS infection to occur.

MANAGEMENT

Several methods have been developed for bacterial canker management in sweet cherry orchards. The best approach is an integrated one that considers cultural practices, cultivar and rootstock decisions, and biological and chemical controls. In WA, efforts should target frost protection, cultivar selection, irrigation strategies, pruning, and sanitation.

Cultural practices
  • Verify new material is clean (i.e., free of cankers) and has healthy buds prior to planting.
  • Avoid planting during frost periods or in cool areas of the orchard. Frost conditions after planting can increase the infection rate.
  • Monitor new plantings during spring and summer for symptoms of gummosis, cankers in limbs, dead buds, or vascular necrosis.
  • After planting, if heading is needed for tree training, wait until the weather is hot and dry and allow the wound to dry before irrigating.
  • Paint the trunk of young trees with white paint to protect from frost or heat damage (Figure 8).
  • If there are open wounds due to frost, rodents, pruning, or other injuries, avoid sprinkler irrigation until the wound is dry or closed.
  • While overhead cooling in sweet cherries is uncommon, it is important to consider the higher risk of open wounds when exposed to water.
  • Prevent exposing green tissue to irrigation: drip irrigate the first years, protect tree trunks with paint or plastic covers, and remove lower shoots or spurs (Figure 8).
  • In susceptible cultivars, summer pruning during dry conditions is recommended. Fall and spring pruning can increase the risk of infection due to a potential coincidence with cooler, moist weather. If winter pruning is needed to promote growth or renew branches, this should be done during dry conditions (Chandler, 1976, Spotts et al., 2010).
  • Excessive vigor due to increased water or nitrogen as well as late pruning (September) can delay dormancy in the fall. Re-growth or green tissue during the fall increases the risk of frost damage due early frost events, and consequently increases the infection risk.
  • Avoid excessive irrigation that leads to runoff to prevent PSS present on leaf surfaces and nearby weeds from being transported through the orchard.
  • When removing tree trunks due to bacterial canker, do not interplant immediately or near infected trees.
  • Remove from the orchard and then burn all infected material including branches or trees.
  • Soil physical-chemical conditions should be monitored and adjusted for adequate growth and free of ring nematodes. Soil limitations such as extreme pH, calcareous layers, shallow water table, among others can weaken trees and increase susceptibility.
  • Early fall defoliation might provide additional control in areas with high risk to early frost damage, by increasing bud hardiness (unpublished data), or by promoting leaf scaring prior to ideal conditions for infection (wet and cool weather).

 

 

young tree with drip irrigation and trunk painted white

Figure 8. Best cultural practice in new plantings of ‘Coral champagne’ on Mazzard, with double drip line and tree trunk protected with white paint.

Physical and Chemical Management Techniques

If P. syringae infection is present in the tree, control of the infection and spread can be done by:

  • Cutting and removal of infected tissue: limbs with gummosis or cankers.
  • Cauterization burning cankers on limbs with a hand-held propane burner. Should be done during spring (Hawkins, 1976, Foss and Antonelli, 2012).
  • Dormant sprays: fixed copper (copper hydroxide, Cu ++) sprays. Copper oxides (Cu+) have shown to be ineffective in controlling the disease (Torres and Latorre, 2009).
  • Pre-bloom: copper sprays + antibiotic based Kasugamycin HCL hydrate
  • Sulfur sprays (Foss and Antonelli, 2012)

When using pesticide products, always read and follow all label instructions. Do not apply copper products after full bloom (Foss and Antonelli, 2012). Note that there are reports of resistance of P. syringae to copper and antibiotics in Oregon (Pscheidt and Ocamb, 2021). Poor control can be due to resistance to pesticides, poor chemical coverage, inadequate timing, and/or high pressure of disease caused by PSS in the orchard.

 

Future alternatives

Much of the infection and high incidence of bacterial canker around the world is closely related to high frost risk areas (southern Chile, Canada, WA, OR), thus control has been mostly directed to frost management (Lindow, 1983). The prospect of cellulose nanocrystals to protect wood and buds against frost damage might offer additional tools to prevent bacterial canker in sweet cherry orchards (Whiting, personal communication).

Another future alternative solution to managing bacterial canker infections in sweet cherry orchards is utilizing trees that are naturally resistant to infection from PSS. While certain cultivars have been reported to naturally have some resistance to bacterial canker infection, the precise underlying genetic factors responsible for infection resistance have yet to be elucidated. Once heritable genetic factors involved in bacterial canker resistance are identified, these factors might then be targeted by breeders to develop new, superior cultivars for sustaining the sweet cherry industry.

USEFUL ADITIONAL INFORMATION FOR WA GROWERS
REFERENCES

Junior AP. 2000. Bacterial Canker and Gummosis of Stone Fruit Trees. University of Idaho Research Extension. Industry Bulletin.

Kennelly MM, Cazorla FM, de Vicente A, Ramos C, Sundin GW. 2007. Pseudomonas syringae Diseases of Fruit Trees: Progress Toward Understanding and Control. Plant Disease 91:4-17.

Latorre B. 1980. Cancer bacteriano, gomosis y tristeza del cerezo. Simiente (Chile) 50(1-2):38-39.

Latorre B. 2008. Cerezos saludables. Especial cerezos. Agronomia y Forestal (Chile) 34:8-11.

Lindow SE, Arny DC, Upper CD. 1978. Distribution of ice nucleation-active bacteria on plants in nature. Appl. Environ. Microbiol., 36. pp. 831-838.

Mgbechi-Ezeri J, Porter L, Johnson KB, Oraguzi N. 2017. Assessment of sweet cherry (Prunus avium L.) genotypes for response to bacterial canker disease. Euphytica 213, 145.

Moore LW. 1988. Pseudomonas syringae: disease and ice nucleation activity. Ornam. Northwest Arch. 12:3–16.

Roche MM. 2001. Development of an in vitro and modification of an in vivo bioassay to screen cherry genotypes for response to inoculation with Pseudomonas syringae pv. syringae. Thesis. Oregon State University.

Romoli, M. 1963. Situación de las investigaciones sobre el cancer bacterial de los frutales de hoja caduca. Simiente (Chile) 33(3-4):25-28.

Spotts RA, Wallis KM, Serdani M, Azarenko AN. 2010. Bacterial canker of sweet cherry in Oregon-Infection of horticultural and natural wounds, and resistance of cultivar and rootstock combinations. Plant Disease 94:345-350.

Torres  R, Latorre B.  2009.  Efectividad  de  compuestos  a  base  de  cobre  en  el  control  de Pseudomonas syringae pv syringae en ramillas de cerezo. Resúmenes  Jornadas Agronómicas.

Hawkins JE. 1976. A cauterization method for the control of cankers caused by Pseudomonas syringae in stone fruit trees. Plant Disease Reporter. 60:60-61.

 

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