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Understanding Alternaria leaf blotch and fruit spot

Pest and Disease Management

During the 2023 APAL Technical Symposium last month, Associate Professor Femi Akinsanmi presented his research on Alternaria leaf blotch and fruit spot. Femi investigated the life cycle and management of the disease but also shed light on potential post-harvest expression of fruit spot in soft-skinned cultivars.

Why do we worry about it?

Australian research studies have shown that the annual production loss caused by Alternaria species was between 15-25% in high value cultivars.

Serious infections can also defoliate trees resulting in an overall reduction in plant health for following seasons. There are also reports of some cultivars expressing the disease shortly after being packed and arriving on supermarket shelves. With fruit showing symptoms of the disease likely downgraded for juicing, which results in a significant financial impact (90% reduction) to the growers.

What does is look like?

In Australia, leaf blotch and fruit spot are actually caused by several different Alternaria species interacting together, with each species behaving slightly differently to each other. For example, some species are more aggressive than others in terms of leaf damage or fruit infection. In addition to this, each growing region likely has its own unique mix of these species that may cause the disease to behave differently in the way to other regions which has control implications for growers.

Leaf blotch symptoms express as roughly circular, reddish-brown spots or lesions on leaves often surrounded by a purple/blackish border. The damage on fruit appears as small (±2 mm), brown, sunken spots on the surface of the fruit and often show a black border, quite often around lenticels.

Figure 1

Life cycle and spread

Like most other diseases the amount of damage or pressure we see during the season depends highly on the weather and inoculum load present in the orchard. Most of the spores present in the orchard are in leaf litter, fallen fruit and old prunings waiting for the right environmental triggers to germinate. However, in other countries the disease is also found on the tree in growing points such as twigs and buds.

The primary mechanism for spread is rain splash, causing spores to fly upwards from the ground and onto the lower canopy. As the infection progresses, it will naturally move to the top of the canopy as more leaves get infected and carry spores higher. Although other forms of disturbance like spraying may allow spores to travel further up the tree.

Young leaves are very susceptible to the disease when compared to mature leaves, so timing protective sprays with leaf flushes will help keep the pressure down through the season. Spring is the typical timing for the initial infection, but we don’t see the typical leaf blotch symptoms until around 40 days after bloom.

It’s important to get on top of this initial infection early because the disease will be present the whole season and keeping the pressure low is key to reducing the risk of fruit damage and defoliation, which isn’t seen until around 110-120 days after bloom. However, unlike the leaf scenario, fruit is susceptible at all growth stages and usually comes in closer to harvest.

Figure 2

How do we manage it?

In Australia, control is mainly achieved with the use of fungicides and luckily many spray that are used for the control of Apple and Pear Scab also are useful for managing Alternaria spp. Yet less is understood on how the mix of several species may have an impact on timing and efficacy of fungicides.

Femi’s research showed that destroying crop residues by burying, mulching and sweeping was the most effective for reducing inoculum levels and improving spray efficacy. Which is the same message that was covered in the recent APAL webinar with Dr Peter Triloff and Fanny Le Berre (RIMpro) for managing Black Spot.

The solution? Reduce the inoculum.

Given the majority of the overwintering spores are located in leaf litter on the orchard floor, examples were given from vacuuming inoculum with specialised implements, to blowing to pick up leaves to the simple sweep/rake and mulch.  The key message was that a lower inoculum orchard will have less disease pressure, increasing the likelihood of adequate control.

To add to this, Femi had one more take-home message for growers and that was to focus on improving tree health. Nutrition such as Magnesium, Manganese, Sulphur and Zinc are key to building healthy leaves and the healthier a plant is, the better it is at resisting infection. So, continue to monitor your leaves for signs of deficiency and speak with your local crop advisors about the best timing for leaf and fruitlet to stay on top of your orchard nutrition.

APAL thanks Associate Professor Femi Akinsanmi for sharing his knowledge and research with us and he also kindly offered to share his presentation here for additional information.

Also please remember that if you have a suggestion for a webinar topic. Feel free to get in touch with Nic Finger [email protected] with your suggestions.

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