Oca : Pest & Disease-Free - Not!

Another of the things that is often repeated about Oca is that it is immune to disease and insect attack. Well, that would be nice, but deeper investigation reveals that in its home Andean habitat, it suffers serious attack by assorted nematodes, tuber borers, fungi and viruses. But the question is, how does it fare when faced with our native UK pestilence?
So here are the main problems I've encountered:


Slugs
Slime trails and munched leaves. Yes, slugs will eat Oca, but I find it to be quite rare. All of that oxalic acid in the leaves is a natural defence, and they seem to move on after a few leaves at most.



Unidentified Leaf Lurgy
I think this is some kind of rust fungus. It usually only appears on plants that are stressed by heat and lack of moisture, and the plant recovers given more favourable conditions. Those growing in light shade do not seem to suffer this problem to the same degree.

Frost
Not a pest, just frost-damage. The outer leaves have been killed by a light air frost, but the stems are undamaged.
See also here for more on frost damage to foliage, and here for tuber damage.

Unidentified Stem-rot
This stem rot occurs at ground level, usually just browning the stem, but occasionally withering it all the way through, causing the foliage to die. I've seen this every year to some extent, so it may be something that survives my composting process, or which is permanently present in the soil.
I've seen healthy and diseased stems right next to each other without it spreading, although on one occasion I've had a (weak) plant completely killed. It looks similar to potato blackleg.

Blackfly
Now and again I've seen blackfly on stems and leaves, but they have never stayed long, usually moving on to some nearby preferred venue, such as broad beans or peas, so presumably they don't like the taste of Oca.
Although they don't do too much direct harm, there is always the concern that they may carry viruses from plant to plant, so as a precaution I squash 'em on sight.

Rats & Birds
I've had an instance of rats and birds (I think ring-necked parakeets) scratching up and damaging tubers during very hard weather. I think this was only because of the desperate conditions, but it is worth watching out for swelling tubers pushing themselves up out of the ground where they could be an obvious target for hungry vermin.


So, quite a short list compared with diseases of, say potatoes. Though of course the list is probably not complete yet!

High Summer Miscellanea

A few things of interest that caught my eye while patrolling the plot the other day:

Tomato, De Barao Black (thanks Toad) with Oca growing at its feet. It's my first year with this variety, and it's turning out to be very productive - the canes are buckling under the load. The taste is slightly lacking in acidity, but it's good for cooking. I'll be saving seed and probably adding it to my 'grow every year' list.

That's Tigerella (also known as Mr Stripey) with Oca, as usual, providing ground cover. I've already demonstrated that tomatoes and Oca grow well as a bi-crop, and it's working just as well for me again this year.

The first of this year's Yacon flowers with a hoverfly getting stuck in. I'll be watching for seed setting, but like Oca, this is another awkward outbreeding blighter when it comes to pollination - this time because male flowers don't appear until after the female flowers, and even then, seed set is said to be poor.

A domestic bee and a bumblebee doing their thing on a globe artichoke. The plot is literally buzzing with pollinators this Summer. This is partly because we have beekeeping on the site now, but also the increased use of organic methods by plot-holders seems to have boosted the general insect population. This is all good news, especially for those of us aiming to collect seed from difficult-to-pollinate crops like Oca or Yacon.

This is Ulluco, which I'm growing for the first time this year. Having now seen its growth habit, it seems another strong candidate for ground-cover in a vegetable polyculture. It's lower-growing than Oca, and fills out as ground cover a bit earlier in the season. I could see it working well with leeks, corn, chili-peppers, tall peas, tomatoes...
But first, I need to obtain tubers from this year's crop, which is by no means guaranteed from all accounts.
Update: Harvest results here.

Other gardeners have squashes growing out of their compost heaps ...
Hats off to Oca, a resilient survivor - last year's dross tubers have sent stems struggling through the 3mm wide aeration holes of this plastic compost bin despite being buried under two feet of mouldering vegetable peelings.

The All-Tuber Polyculture Mound

In a quiet corner of 'my other plot' I'm trying another cultural method - a four foot wide mound with three different tuber crops grown closely together. I'm aiming for a low-maintenance easily-harvested dense ruck of tuberous productivity.

Unlike standard polycultures, a clear requirement for an all-tuber polyculture is that the crops involved should mature, and be harvested all at the same time, otherwise lifting one will disturb the roots of those remaining.
Oca, Yacon, and Chinese Artichokes together satisfy this criterion quite well, all normally being harvested after their top growth has been killed by frost.

I started in Spring by clearing the area of the previous Jerusalem artichoke crop (yeah, right!), and digging in a barrow-load of rough compost. This was more to improve water retention and aid soil friability (and hence make harvesting easier) than it was to boost fertility. On top of the mound goes one of my prized new variety purple Yacon, purportedly quick-maturing, but so far untried by me. It is certainly much more vigorous than my other 'standard' Yacon variety. Around this goes Oca, then on the outside edge are the Chinese Artichokes.

This little experiment could easily be scaled up to an informal linear raised bed (or 'lazy bed') if one wanted. It could even work on a commercial scale if suitable harvesting machinery was available.

With all the incorporated compost, thorough deep cultivation, and dense weed-suppressing foliage, this is also ideal as a once-and-for-all soil improvement method before turning ground over to no-dig culture.

Drought is the problem this season. All three crops are showing stress, but it will still be interesting to see what quantity of tubers can be got from this single square metre of ground.

Update 31/8/10

The drought has given way to a couple of weeks of pleasant showery weather. The soil moisture, no doubt helped by all that compost in the mound, has caused the Yacon to double in size. It is now seven foot wide and tall, topped with a lanky bouquet of flowers.
It is even suppressing a couple of late-breaking jerusalem artichoke volunteers, and I now fear for the productivity of the Oca and Chinese artichokes.
I either need a smaller Yacon, or a larger mound!

Update 1/11/10

The first air frost on 21/10 burned back the yacon foliage, but has not completely killed it. Under its protective canopy, the Oca plants have escaped damage, unlike those planted on open beds nearby.
A few days before the frost, I noted the yacon had grown to have a spread of nine feet!

Update 12/12/10
There has been freezing weather for a couple of weeks, and the plants are showing no sign of life. It's a dry day, so a good opportunity to exhume the contents of the mound.

First up is the Yacon. It's a big one! The Health & Safety Executive would have me use a hoist for this job, but after a bit of grunting I manage to get the crown in to a wheelbarrow solely by manual handling methods.

After washing, the useable tubers weigh in at 18 lb (8.2 kg), with another pound or two of small or damaged ones.

Delving around nearer the edges of the mound reveals the 'also rans' – a moderate scattering of mostly undersized Oca and Chinese artichoke tubers.

About 1.5 kg in total – which is as much as I can expect given that the plants have been camped under dense Yacon foliage for most of the growing  season.

CONCLUSIONS
This growing method was successful in terms of yield and low-labour, despite an unusually early frost. Next year it will be even easier; it will only be necessary to plant the Yacon, as there should be ample volunteer Oca and Chinese artichokes.

The imbalance in yield between the three crop species was caused by misjudging the vigour and final size of the particular variety of Yacon chosen. I mistakenly assumed it would be similar to the 'standard' Yacon that I have grown previously, and as a result the Oca and particularly the Chinese artichokes suffered from lack of light. However, a variety being too successful is a good fault, as they say.

It is an interesting and potentialy useful observation that the Oca were protected from the first frost by the Yacon foliage. Yacon, not being reliant on day length, can tuberise early, before sacrificially protecting undergrowing tender crops which have yet to fully tuberise. The partial defoliation lets through more light to ground level, and as long as there is not a second frost, the lower crops benefit.