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Biological fungicides: A viable complement to chemical fungicides

Pest and Disease Management

Dr Gordon Brown discovers more about Bacillus subtilis and reviews research on its potential to control crown rot.

As we transition away from chemical pesticides to more environmentally friendly alternatives, we need to become more aware of the alternatives available, how to optimise their performance and what their limitations are.

Bacillus subtilis has been used overseas for the control of many diseases and is now commercially available in Australia. What is known about this bacterium and how does it work? Its potential was demonstrated in a recent scientific paper on crown rot (Phytophthera cactorum) control in apples.

Figure 1: A young apple tree showing symptoms of dieback due to Phytophthora root rot.

What is Bacillus subtilis?

B. subtilis is a bacterium normally found in the soil and in the intestinal tract of animals and humans. Because of this, there is no withholding period when using this bacterium for disease control. There are many strains of B. subtilis and strain names tend to be codes assigned by bacteriologists when they find an unknown bacteria in one of their samples. A strain code is, in essence, similar to a cultivar name of an apple or pear and there are subtle differences between different strains. Traditionally there has been a strain of B. subtilis known as amyloliquefaciens; however, in recent times this has been renamed to Bacillus amyloliquefaciens. It differs from B. subtilis in that it produces the enzyme amylase that breaks down starch. Otherwise, it is very similar to B. subtilis and the United States Agricultural Research Service Culture Collection lists the two as being synonyms of each other. This helps to explain why this bacterium is often quoted as being B. subtilis in overseas research papers and on product labels.

Another bacillus useful to agriculture is Bacillus thuringiensis, which has been successfully used since the early 1970s as a control for leaf-eating caterpillars. Originally it went under the trade name of DiPel®, but it is now available under a number of different trade names. Of interest, there are eight strains of B. thuringiensis registered with the APVMA for the control of caterpillars in Australia and it can be expected that many strains of B. subtilis and B. amyloliquefaciens will become commercially available.

What does Bacillus subtilis/amyloliquefaciens do?

A scientific literature search has identified sixty-seven papers involving the use of B. subtilis/amyloliquefaciens on apples or pears. Of these, there are forty-two papers covering the control of twenty-two diseases by a range of strains. Diseases include foliar infections such as black spot, anthracnose and powdery mildew; root diseases such as Phytophthora, Fusarium, crown gall and white root rot; and diseases of the fruit during storage such as penicillium, bitter rot and Alternaria. A common theme in the research papers is that B. subtilis/amyloliquefaciens on its own does not completely control a disease, but assists other materials such as pesticides and other bioactive products to achieve commercially acceptable control of the disease.

An example of this is that B. subtilis (QST 713) can be used to successfully replace broad-spectrum cover sprays of Captan or Mancozeb in a spray program against Black spot where other more targeted pesticides, such as SDHI fungicides, are also used. Another finding is that the control achieved with B. subtilis/amyloliquefaciens is reduced if the disease is already established in the tree. In other words, B. subtilis/amyloliquefaciens is best used as a preventative treatment rather than a curative one.

Hence B. subtilis/amyloliquefaciens can be viewed as a non-chemical broad-spectrum fungicide with modest efficacy. Modes of action identified have included crowding out the pathogen, production of antibiotics and other fungistatic compounds, and stimulation of the plant’s own defence system.

Review: The impact of B. subtilis on Phytophthora collar rot in apples

A scientific paper was recently published on Turkish research (Kaymak 2022) where eleven different biological products were compared in pot trials for their efficacy against Phytophthora cactorum, responsible for root and crown rot of apple trees. For this review I will focus on the two commercially available B. subtilis products, Companion® and Subtilex® that were used in these trials. Companion is available in Australia and typically contains 1.5 x 107 spores. Subtilex is not available in Australia, although BASF have trademarked the name in Australia and their Serifel® product contains the same strain of bacteria at the same concentration (5.5 x 1010 spores) so it is possibly the same product. It should be noted that Subtilex has 55 billion spores compared to 15 million spores in a gram for the Companion product.

In this trial, orchard soil mixed with sand and Phytophthora cactorum was used and young nursery whips were planted into each pot prior to treating with Subtilex or Companion at the label rates 0.5ml/m2 for both. The trees were then grown for 4 months prior to measuring the degree of Phytophthora collar rot disease severity (percentage of stem collar with lesions) and tree growth characteristics. This trial was repeated in the following summer to confirm the results.

In this trial, the trees were not treated with B. subtilis treatments to allow for colonisation of the roots and stem prior to exposure to Phytophthora. Hence this was a severe challenge for the products, and superior control of Phytophthora may be expected from earlier applications of the products. Despite this, it was found that both the products reduced the degree of Phytophthora infection from 45 per cent of the stem collar with lesions to less than 10 per cent ­– a substantial improvement (Figure 2).

As expected, this reduction in disease severity was accompanied with an increase in the dry weight of the roots and the terminal shoot length. Despite the substantially lower number of spores applied with the Companion product, the growth of the bacteria on the roots was sufficient to inhibit Phytophthora development to a similar extent to the Subtilex, although the growth promotion was not as great. This difference in growth performance of the two products, despite a similar level of disease control, may be related to differences in the growth promotion potential (microbial fertiliser activity) of the two different strains of bacillus.

Figure 2: Disease severity and growth performance of young apple trees infected with Phytophthora cactorum 4 months after treatment with Bacillus subtilis products.

The authors of this study concluded that of the eleven biological control products tested, both Subtilex and Companion were among the top four for control of Phytophthora cactorum in these trials.

It should be mentioned that the other products tested in the trial included Symbion Vam (Glomus fasciculatum; Stanes), Green Miracle (Vegetable oil acid; Stanes), Cropset (Lactobacillus acidophilus, plant extract, MnSO4, FeSO4, CuSO4; Improcrop EU), Isr-2000 (Lactobacillus acidophilus, plant extract, yeast extract, benzoic acid; Improcrop EU); Actinovate (Streptomyces lydicus; Mts agrol), Tricho plus (Trichoderma harzihanum; Bioglobal), Alexin 95 PS (Phosphorus pentoxide; Sumitoma), Combat Plus (Plant activators; Bioglobal) and  Endo Roots Soluble ( a mycorrhizal product including various Glomus species + Gigaspora margarita; Bioglobal). Of these Alexin 95 PS and Endo Roots Soluble reduced the percentage of disease severity as much, or better than, the two Bacillus products in the pot trial.

Further reading

Kaymak S (2022) “Effects of some commercial products on root and crown rot caused by Phytophthora cactorum in apple cultivation”, Turkish Journal of Agriculture and Forestry, 46(1).

This is an extract from Gordon Brown’s research review in the forthcoming Spring 2022 edition of AFG. Research reviews are commissioned by APAL as part of its Industry Services.

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