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Stimulating the tree’s defences against black spot

Pest and Disease Management

Black spot (apple scab, Venturia inaequalis). Photo: Shutterstock.

Dr Gordon Brown looks at innovations in biological control of black spot.

Black spot (apple scab, Venturia inaequalis) control is a problem all over the world, with any visual symptoms on the fruit leading to market rejection of the fruit and a financial loss to the grower.

While disease-resistant varieties have been bred, their incorporation into industry has been slow with the market, in Australia, dominantly focused on Gala, Granny Smith, varieties sold as Pink Lady and a handful of club style varieties that are not resistant to black spot.

Many fungicides have been developed and commercialised against black spot, however environmental and health concerns by the public are placing pressure on industry to develop safer, environmentally friendly methods of controlling black spot while still providing quality fruit.

This has led to a search for alternative biological approaches for black spot control in Australian apple and pear orchards.  Among the potential methods are compounds that enhance the tree’s natural defences against black spot, and some have already been commercialised.  One of these is a synthetic compound, Fosetyl-Aluminium (Aliette) which is reported to be an effective compound against black spot although it is not registered for this use at this stage in its development.  This material is currently registered as a foliar spray against collar rot in apple trees with two applications during the growing season.  This material has both fungicidal and defence-stimulating activity.  Another class of materials being investigated are naturally occurring carbohydrates including various forms of sugars.  Most of these have the advantage of being very safe, cheap and naturally occurring.

This research paper summarises findings from European research into the effectiveness of two fructans – one of which has been found to be effective against botrytis on lettuce – against black spot in apples, and compared to Fosetyl-Aluminium, reported to be the most effective plant defence stimulator found for black spot to date.  See further reading for full paper reference.

Fructans are a sucrose molecule (a glucose ring connected to a fructose ring) with additional fructose rings attached.  Fifteen per cent of plants produce fructans, so these are definitely naturally occurring compounds.  They are known to accumulate in plant species that are subject to abiotic stresses.  This paper studied the activity of a group of fructans called levans and another called inulins.  See Figure 1 for the structural differences in these molecules.

Figure 1.  Structural differences between inulins and levans.  They have the same chemical formula, but they are a different shape.

 

In this trial the inulin used was derived from chicory roots and purchased through a scientific supply company.  The levan was extracted from Dactylis glomerata (Cocksfoot, a common pasture grass) although it should be noted that levans are used in the food, cosmetics and medical industries so industrial quantities are already available for these industries.  The reason that the authors extracted their own levan is not known, although it is noted that the current marketing of levan on the internet dominantly includes products that are actually inulin.  Levan is commonly found in monocots and inulin is the dominant fructan in dicots.  Added to this extraction of levans from grass leaves is simple, soak in a 35g/L sugar syrup for two days in sunshine to cause a stress and stimulate levan production, then extract in 85°C water for 15 minutes.  Purification, if required is more complex.  Both these fructans were sprayed onto the top three leaves of young apple seedlings at 1g/L. Fosetyl-Aluminium was also sprayed at 1.25g/L, this is a little over half the label rate of Aliette in Australia for apples against phytophthora collar rot.  After three days the plants were sprayed with a black spot spore suspension and then the infection process was observed over the following 16 days.  In addition, these materials were incorporated into agar and the growth of black spot on the agar plates measured as an indicator of straight fungicidal activity independent of plant defence responses.

After inoculation the leaves of the trees were kept wet in the glasshouse to encourage infection and disease development.  Averaged across the three leaves the level of control achieved with the inulin was identical to the untreated control indicating no disease control on the trees for this material.  The level of disease control achieved with the levan was excellent and similar to the Fosetyl-Aluminium control, the most effective defence stimulator to date (Figure 2).  These results are reflected in the quantity of black spot DNA recovered from the leaf surface.  No foliar damage due to the fructans was reported.

Figure 2. Percentage of leaf surface covered in sporulating black spot lesions 16 days after infection and the quantity of black spot DNA recovered from these leaves.

 

The growth on agar plates (Figure 3) indicates that the presence of both fructans at the rate used on the leaves in the agar reduced the growth of black spot indicating a direct fungicidal activity of both fructans, although they were possibly not quite as effective as the Fosetyl-Aluminium.  Looking at the graph in the original publication it appears as though Fosetyl-Aluminium at a high rate is extremely effective compared to the fructans, however the high rate of Fosetyl-Aluminium reported there was 10 times stronger than that used on the leaves and according to the Aliette label it is liable to damage the trees if used at this rate.  The highest rate of the fructans used in this agar plate trial was the same as that used on the leaves so be careful if studying the original graph.  The reason for incorporating the high Fosetyl-Aluminium rate in this trial is not clear.  The authors provide a possible explanation for the inulins reducing black spot growth on the agar plates and not on the trees, as levans might be more effective than inulins at stimulating apple tree defence mechanisms.  They also indicate that inulin might be used by black spot as a food source.

 

Figure 3.  Growth of black spot on agar plates treated with fructans or Fosetyl-Aluminium.

 

 

Take home message

This study has identified a cheap and sustainable natural product similar to sugar that is normally found in plants which provides similar control of black spot to Fosetyl-Aluminium, a synthetic fungicide with reportedly good activity against black spot.  It is considered that this control is due to both a mild fungicidal activity as well as stimulating the trees inbuilt defence mechanisms.  Further research is needed to confirm levan efficacy against black spot in field grown trees and to develop optimal application techniques.

 

Further reading

Sweet immunity: the effect of exogenous fructans on the susceptibility of apple (Malus × domestica Borkh.) to Venturia inaequalis.

Svara, A.; Tarkowski, Ł. P.; Rensburg, H. C. J. van; Deleye, E.; Vaerten, J.; Storme, N. de; Keulemans, W.; Ende, W. van den. (2020)   International Journal of Molecular Sciences  21:16

 

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