Managing pear scab (black spot)

On 2 November Marcel Veens joined us online to discuss the options for managing pear scab in the lead up to harvest, based on an understanding of the disease cycle and how using tools that monitor weather conditions can help with the timing of application of sprays more effectively than ‘calendar’ sprays.

Watch a recording of the online discussion.

Key points

• Control early (Marcel says “Protectant, protectant, protectant”) to reduce the initial inoculum
• Forecasting tools can help with decisions around when to spray
• Application of the appropriate sprays leading up to harvest can help to manage post-harvest summer spot
• Orchard hygiene can help reduce carry over inoculum from one season to the next

Scab lesions on pear. Photo: Bruce Watt, University of Maine, Bugwood.org

What is pear scab?

Pear scab is caused by the fungus Venturia pirina, which infects leaves, fruit and shoots. Symptoms can be seen on infected leaves and fruit as olive green, brown and black, velvety spots. As the lesions become older they become scabby and cracked. Infections that occur late in the season may not be obvious until after the fruit has been stored. These lesions are much smaller and the disease may be referred to as summer spot.

Early control is best

The pear scab fungus overwinters in infected leaves remaining on the ground from the previous season. The first spores (ascospores) of the season are produced in these leaves and released during spring coinciding with the development of buds, flowers and leaves. For infection to occur, plant tissues must be wet for 10–25 hours.

Forecasting tools

Control at the primary infection stage is key and monitoring the weather conditions can help to target spray application to the optimum time to reduce the risk of infection. Using decision-making tools can also help to know when conditions are suitable for infection to occur – Marcel says that if you can see symptoms, the infection took place 2–3 weeks ago. Reducing the initial inoculum levels can help reduce the number of infections and production of secondary inoculum.

Spores (conidia) produced from these initial (primary) infections go on to infect more leaves and fruit, including after the ascospore production has finished. Many secondary infection cycles can take place throughout the season if conditions are suitable. This means that infection can take place all the way through to harvest.

A pear showing symptoms of the pear scab disease caused by Venturia pirina. Photo: Howard F Schwartz, Colorado State University, Bugwood.org

Coming up to harvest

Marcel says fruit are more susceptible to infection when young, but even mature fruit can become infected. This is an important consideration coming up to harvest. Remembering that the time between infection and the appearance of symptoms is around 2–3 weeks. This means that fruit that is infected shortly before harvest may have no obvious symptoms, but symptoms can develop later during storage. Application of appropriate control measures in the weeks leading up to harvest can help to manage the risk of infection and development of post-harvest symptoms.

After harvest

Remember, the fungus can survive on leaves on the ground in the orchard and on wood. Remove pruned material. Mulch leaves and apply urea, molasses and other approved products to help break down leaves and reduce the inoculum carry over. This will help to reduce the starting inoculum for the next season.

More information about pear scab and apple scab can be found here:

• Agriculture Victoria: plant diseases – pear scab
• Agriculture Victoria: plant diseases – apple scab
• WA DPIRD: Managing apple scab in Western Australia
• An example of chemicals approved for apple and pear scab can be found in Table 73 on page 98 of the Orchard plant protection guide for deciduous fruits in NSW 2022–23
• “Pear Scab in Oregon — Symptoms, disease cycle and management” by RA Spotts and S Castagnoli, Oregon State University, 2010

The information provided here and in the linked presentation is general in nature. Please seek professional advice. Please check product labels for recommended use and application guidelines.