Worm Castings – Part IV – Disease Suppression

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Hopefully I didn’t frustrate to many people with all the scientific article references in the last newsletter.

If I did, let me apologize in advance, since there will be some more coming your way this week. 🙂

As promised, we’ll be looking at worm castings as agents of plant disease suppression.

Over the years there has been considerable research examining the disease-suppression potential of thermophilic (hot) composts, but relatively little, in comparison, conducted using vermicomposts (again, recall that we are basically using this term as a synonym for ‘castings’).

Nevertheless, there is still a respectable (and growing) body of evidence to indicate that castings do indeed possess properties that can help to protect plants against a variety of diseases.

This past fall I was fortunate enough to have the opportunity to interview (for an article on my website) Allison Jack, a PhD candidate at Cornell University who has been looking into the area of compost-mediated plant disease suppression for her thesis project.

One component of her research has centered around the suppression of the ‘damping off’ pathogen, Pythium aphanidermatum, brought about via the addition of dairy manure worm castings (Tom Herlihy’s “Worm Power” brand). Specifically, she has been trying to determine what mechanisms are responsible for the significant reduction in disease incidence when castings are added to the soil blend.

One thing that has been determined without much doubt is the fact that the microorganisms in the castings are primarily responsible for the disease suppression – at least in this particular case.

When castings were heat sterilized before being added, most of the seedlings were killed by the pathogen – similar to treatments without any castings added.

What makes this even more interesting is the evidence suggesting that this microbial protection is not the result of the ‘good guy’ microbes attacking or even out competing the pathogen – rather, it seems to be due to some sort of alteration of chemical cues.

Pythium colonizes seeds/seedlings via swimming ‘zoospores’, which rely on chemical cues in order to find their host, so when these cues are no longer present the guidance system is basically thrown off and many of the zoospores never reach the seeds.

One of the real challenges in this field (and compost science in general) is that not all castings are created equal. Aside from the basic ‘good’/’bad’ quality considerations, there are so many different variables to consider – often making it difficult to draw reliable parallels between different studies (since different castings are typically used).

Something Allison pointed out is that the starting material (the ‘waste’ used to make the castings) itself can be a very important consideration – in other words, just because her ‘dairy manure castings’ effectively suppressed Pythium in cucumber seedlings, that doesn’t necessarily mean that food waste castings would have produced the same results, or that the manure castings would be as effective against other diseases etc.

A study conducted by Szezch and Smolinska (2001) demonstrated that different starting materials can result in significantly different worm castings (in terms of disease suppression). The researchers tested four different types of castings – three using cattle, horse, and sheep manures, and one using sewage sludge – in order to assess their ability to suppress Phytophthora in tomato seedlings.

While all of the manure castings did indeed provide significant protection against the disease, the sludge castings offered no protection at all. What’s interesting is that, similar to Allison’s findings, the manure castings did not actually reduce the abundance of the pathogen, which seems to once again indicate that there can be mechanisms at work other than direct attack/competition.

One thing that is very important to note about this particular study however is the fact that the sludge contained three times more zinc than is legally permissible in soil, and the material was shown to hamper the growth of the plants in general (and even brought about a reduction in pathogen abundance – although this didn’t help reduce incidence of disease). This is a prime example of why it is always important to look a little deeper, and not to draw any major conclusions (i.e. ‘sludge castings are bad’) from the results.

In an earlier study by Szczech (1999), cattle manure castings were found to significantly inhibit the infection of tomato plants by Fusarium. In this case however, direct antagonism against the pathogen seemed to play an important role. Microscopic examination of the pathogen organisms in treatments where castings were applied revealed that the Fusarium had been attacked and colonized by other microbes.

Direct antagonism also seemed to play a major role in the disease suppression reported by Utkhede and Koch (2004). The researchers found in this case that applications of worm castings tea resulted in as much as 63% reduction in bacterial canker incidence in tomato plants.

We’ll certainly be talking a lot more about castings tea in future newsletters, but I thought this was an interesting finding to mention, and it’s certainly applicable to the present discussion.

While all of the above-mentioned studies (and others) do seem to indicate that castings can potentially offer effective stand-alone protection against plant diseases, I came across a couple of other interesting research papers that demonstrated that the use of castings in combination with other disease suppression agents can be even MORE effective.

Sahni et al. (2008a), showed that castings used in combination with biocontrol bacterium, Pseudomonas syringae, resulted in the highest level of protection against ‘collar rot’ in chick peas. Interestingly enough, Sahni et al. (2008b), showed that the use of certain chemical agents along with the castings and the same biocontrol organism provide more protection still.

I wanted to mentioned these last two studies, since they highlight the importance of not necessarily thinking of castings as the ‘be all, end all’ solution – but rather as one (potentially) effective component of a multi-pronged, integrated approach. The same can be said of their growth promoting abilities – as the OSU research (discussed in a previous newsletter) seems to indicate, the best results are achieved when castings were combined with conventional fertilizers and methodology.

As per usual, there is plenty more that could be said about the disease-suppression potential of worm castings, but alas I have already stretched this article to the limits (and then some)!

It’s no secret that the topic of worm castings in general is one that I find very interesting! Hopefully all our readers won’t mind at least one more installment in our castings series (did he just say ‘at least’?! haha), before we move on to the topic of ‘worm tea’.

Stay tuned…

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