Thursday, 18 November 2010

Oca as Soil Fungicide?

I've previously proposed that Oca makes a good bicrop partner for outdoor cordon-grown tomatoes. This has been based on..., the fact that it seems to work well for me, certainly in terms of physical compatibility - space, light and soil utilisation.

Cue photographic propaganda:-

Now, I must say that when it comes to plant companion effects I am, if not actually sceptical, then at least untroubled by high expectations. Some traditional companion effects do not stand up to objective tests, and when they do, it is such a multi-factorial situation that it's difficult to say if the results will successfully translate to other growing conditions.

However going back to Oca and tomatoes, I recently found this paper which shows that water-soluble extract from Oxalis articulata foliage can suppress the soil-borne fungus Fusarium oxysporum, or as it is better known, Fusarium wilt, a particularly destructive and persistent disease of many crops, including tomatoes.
We only have to make a small speculative step to assume that other Oxalis species (including Oca) will contain the same natural fungicide, and by planting them with tomatoes there may be some protective effect  during the growing season.

Thinking more, this also supports my normal practice of leaving oca crop debris to cover the soil after harvest. Fusarium spores can survive in soil over winter, ready to infect any suitable host crops the following summer, but decaying Oca foliage may be releasing natural fungicide, to be rinsed into the soil by winter rain.

My outdoor tomatoes die from various things, usually late blight, but one year some, dutifully interplanted with French marigolds, died of fusarium wilt. Now all I'm saying is,  perhaps if they had been interplanted with Oca instead...

UPDATE 23/11/10
Lab tests reported here show that ocatin (a protein in oca tubers) suppresses fusarium oxysporum, as well as Phytophthora cinnamomi ('dieback' or root rot), and Rhizoctonia solani ('damping off' and brassica wire stem).
Of course, the fact that the tubers can protect themselves from some fungal problems is a separate issue from foliage anti-fungal properties, but interesting nonetheless.