What is a Pear Tree Black Spot, and Why Should You Care?
Pear tree black spot, caused by the fungus Alternaria alternata, is a common disease that affects many home and commercial pear varieties.
The fungi overwinters in fallen leaves and stems on the ground. In spring, spores are produced and spread by rain splash and wind to infect young, developing leaves and fruit.
Signs and Symptoms to Watch For
The first signs of pear tree black spot include small, circular lesions on the leaves. These lesions are flat with concentric rings and measure 2–5 mm in diameter.
The lesions are black, gray or dark brown in color. As the fungus develops within the lesions, spores are produced, which can lead to further infections.
Left untreated, yield and fruit quality may be reduced. Severe infections can cause premature defoliation of the tree.
When Does Black Spot Usually Appear?
Black spot is favored by prolonged periods of wet or humid conditions in spring and early summer. This allows for optimal spore production and dispersal.
Homeowners should scout trees after rainfall or heavy dew periods in May through July for the first lesions.
Young, expanding leaves and developing fruit are most susceptible. Later in summer, spots may also develop on the fruit, making it unmarketable.
Diagnosing Pear Tree Black Spot Versus Other Lookalike Diseases
It’s important to correctly diagnose black spot to implement the proper control methods. Other fungal diseases, such as scab, can have similar symptoms. To distinguish black spot:
- Examine lesions. Black spot lesions are small and circular with concentric ring patterns. Scab lesions are raised, felt-like and larger.
- Check time of infection. Black spots infect young leaves and fruits early in spring. Scab may infect throughout the growing season.
- Observe spore production. Place infected leaves in a zip-lock bag for 1-2 days. Black spot leaves will produce a powdery mass of gray spores. Proper identification leads to better-integrated pest management.
Nipping Black Spot in the Bud With Cultural Controls
Cultural practices promote tree health. They also reduce disease. These practices are the first line of defense against black spots.
- Prune and remove debris in the fall and winter. This removes overwintering spores from infected leaves and stems.
- Plant-resistant rootstocks. Some rootstocks have improved tolerance to black spots, like the new varieties ‘Puxxoo’ or ‘OHxF 87’.
- Monitor plant spacing and air flow. Avoid overcrowded trees and allow sunlight and air movement through the canopy.
- Water wisely. Avoid overhead irrigation, which wets foliage for long periods, favoring disease.
Cultural controls alone may provide enough protection in low-disease years. But they should be combined with other methods in high-pressure situations.
Fungicide Options for Suppressing Black Spot
When preventative cultural practices prove insufficient, targeted fungicide applications throughout the growing season are necessary to prevent significant leaf and fruit infections. Both organic and synthetic options are available.
Organic Options
- Copper fungicides like copper sulfate or copper hydroxide. Apply before infections in spring and after rains throughout summer. High rates may cause phytotoxicity.
- Potassium bicarbonate products like Kaligreen or Armicarb. Works preventatively when applied before infections occur.
- Bacillus subtilis strains in products like Serenade. Shows some efficacy against black spot when applied preventatively.
Conventional Options
- Chlorothalonil (Bravo, Echo) provides excellent control when applied on 7-10 day intervals throughout the season. High resistance risk with overuse.
- Mancozeb (Dithane, Manzate) another protectant fungicide that works well when applied before infections occur and every 7 days.
- Azoxystrobin (Heritage) is systemic and shows good curative activity. Use only 1-2 times per year to reduce resistance.
Proper application timing, coverage and alternation of active ingredients are key to resistance management with any fungicide option.
Monitoring and Adjusting Management Tactics as Needed
Even with proactive controls, black spot pressure may fluctuate annually depending on weather conditions. Careful monitoring is vital.
- Scout trees weekly during high-risk periods in spring and early summer. Examine new growth thoroughly.
- gauge efficacy. Are lesions developing or being suppressed compared to untreated areas?
- Be flexible. If methods prove ineffective, do not hesitate to adjust product selection, add new tactics or increase application frequency as required by disease pressure.
With a well-planned integrated approach and adjustments made based on regular scouting, homeowners can effectively manage pear black spot long-term to preserve tree health and productivity.
Fungicide Options for Suppressing Black Spot
When cultural controls alone are not enough, targeted fungicide applications at key times are useful for black spot suppression. Proper selection and use of products helps prevent resistance issues.
Organic and lower-risk options
Bacillus subtilis: This beneficial bacterium, available in products like Serenade, can inhibit black spot when used preventatively. Apply every 7–10 days during infection periods.
Potassium bicarbonate: Products containing potassium bicarbonate, such as Armicarb or Kaligreen, work best as a protective spray before infections occur. Reapply after rain.
Copper fungicides: Copper sulfate or hydroxide products act as a protectant and may suppress black spot if applied weekly from bud break through summer. Use the lowest effective rates to avoid phytotoxicity.
Conventional fungicides
Chlorothalonil: Bravo and Echo provide excellent control when applied every 7 days, but high resistance risk limits use to no more than 2–3 times per season.
Mancozeb dithane, and Manzate also protect pear leaves well with proper timing. Apply every 5-7 days through infection periods as a resistance management strategy.
Strobilurin fungicides: products containing azoxystrobin, like Heritage, exhibit curative activity against black spot. Use no more than twice per year and always alternate with other modes of action.
Proper coverage, rates and tank mixes are crucial for best results from any fungicide. Rotate chemistry and modes of action for resistance prevention.
Monitoring and Adjustment
Continual monitoring allows orchardists to gauge fungicide performance and black spot pressure levels. Scouts should:
- Examine new growth weekly from budbreak through summer
- Compare treated vs untreated areas and prior years
- Note environmental conditions conducive to disease flare-ups
If signs of control failure emerge, do not hesitate to adjust tactics. This may involve trying different products, increasing application frequency, improving coverage, or integrating additional cultural practices. With flexibility, black spot can be managed sustainably for long-term tree health.
Monitoring Pear Trees for Black Spot Infection
Regular observation and scouting are key to effectively managing black spot on pears. Some signs to watch for include:
- New lesions forming on young leaves, fruits or shoots indicate active infections
- concentric rings and zones of different colors within lesions
- spread of lesions to larger portions of leaves over time
- premature leaf drop or shoot dieback on heavily blighted trees
- spore production is visible as a gray powder on undersides of infected leaves
It’s best to scout trees at least once a week during primary infection periods from budbreak through summer. Some points to check during monitoring:
Examine new growth. Young, expanding leaves and developing fruits are most susceptible to black spot.
Compare disease levels. Note differences between treatment areas and untreated controls.
Observe environmental factors. Conditions like recent rainfall or prolonged leaf wetness can spur fresh infections.
Keep detailed records. Document monitoring dates, pest and disease observations and cultural and chemical controls applied.
Regular scouting allows orchards to identify infection hotspots and gauge fungicide performance. Changes in black spot pressure from year to year also provides insights. With close monitoring, adjustments can be made to refine IPM tactics.
Cultural Tactics for Black Spot Management
Cultural practices that strengthen tree health and disrupt disease cycles are a foundational part of any black spot control program.
Prune and remove debris in the winter /fall. Eliminates overwintering inoculum in fallen leaves/stems.
Use resistant rootstocks. Varieties like ‘Puxoo’ and ‘OHxF 87’ show improved tolerance to black spots.
Consider plant spacing. Avoid overcrowded canopies with poor ventilation and sunlight penetration.
Practice good sanitation. Remove and destroy severely infected plant tissues/fruit as needed.
Adjust irrigation. Avoid overhead systems that have excessively wet foliage and microclimates.
Time orchard activities carefully. Prune in winter vs. spring and avoid crop/tree stress from other pests.
Integrated with other tactics, cultural methods significantly reduce black spot pressure in pears sustainably.
Fungicide Application for Black Spot Control
When infection pressure warrants, targeted fungicide applications help suppress black spot development on pears.
Timing: Apply protectant fungicides before infections occur based on local disease risk periods. For example, in the Pacific Northwest, this is during bloom and early fruit development in spring.
Coverage: Thorough coverage of all new foliage and young fruits is essential for prevention. Use sufficient water volumes (50–100 gal/acre) and add non-ionic surfactants per label.
Resistance Management: Always rotate between fungicide classes and FRAC codes to avoid resistance building up in the pathogen population over time.
Tank Mixes: Consider adding fixed copper fungicides to mixes with contact protectants like mancozeb for their extra curative/protective effect.
Cultural Synergies: Fungicide performance improves greatly when combined with cultural techniques like pruning, proper plant spacing and resistant rootstocks that collectively stress the pathogen less.
Scouting Guidance: Pay close attention to the monitoring results after each application. This enables adjusting rates, coverage or spray intervals as conditions warrant.
With an integrated program comprising cultural practices, resistant varieties, scouting and properly timed/applied fungicides, black spot can be effectively managed on pears for many years.
Conclusion
Effective pear black spot management relies on integrating cultural, biological and chemical tactics according to a farm or home orchard’s specific needs and resources. The keys to long-term success include:
- Regular monitoring is needed to detect black spot early and adapt plans
- Optimizing tree health and resistance through proper cultural care
- Removing inoculum through sensible pruning and sanitation practices
- Applying protectant and anti-sporulant fungicides strategically when needed
- Rotating between fungicide modes of action and FRAC groups
- Considering tolerant rootstocks suited to local conditions
- augmenting controls by manipulating environments and planting beneficial plants
