Grand Theft Oca !

Yesterday this tray contained about 700 true Oca seeds, ...

... the culmination of a season's painstaking hand-pollination and collection. Today it contains two seeds and a few shriveled seed capsules.
They were undergoing their final drying, sitting 'safely' on a table in my work room.  I was planning to offer most of them for distribution or swapping.
Given that there were no signs of breaking and entry, and that no-one in the house has a history of sleepwalking, I was left with the possibility of...  ...Hmmm, there had been rumours of a mouse in the house for a while now.  It didn't seem very likely that it could have climbed the stairs, then the smooth painted steel table, ignored rows of farinaceous delicacies such as dried heritage peas and assorted tubers without giving them a nibble, then polishing off all those seeds in one sitting.
But there was no other possibility,  so it's a job for Little Nipper and a tahini-smeared raisin. The penalty for this crime is death!

Bingo. One rather well-fed mouse! I did seriously consider an autopsy to recover the stolen goods, but I think they would already be mouse droppings by now.

He did leave two seeds, and there are a few more from a final batch of pods yet to ripen, so I'm not quite wiped out, but this is still a massively disappointing setback,

...and another lesson learned.

Wildlife in the Oca, Friend or Foe?

I was initially alarmed by this chap...

...(and several of his mates) browsing amongst my Oca. I watched for a few minutes, and noted that they left the Oca plants untouched, but laid waste to surrounding weeds - a situation that suited me fine. Obviously they qualified as good-guys, and there would be no need for a messy manual squashing session.
It seems it's the caterpillar of the Cinnabar moth. Some research revealed that they are used as a biological control of their favorite food plants, ragwort and groundsel. Apparently after consuming all available ragwort and groundsel in the area, they may become cannibalistic. I do appreciate workmen that tidy up afterwards!

With some Oca flowering, I've been scrutinising stylar arrangements...

... and have noticed a lot of these tiny thrips or thunderbugs in the flowers. They suck sap from plant cells, and are considered a pest. It's possible that they could spread viruses, but on the other hand they may provide some pollination even though they seem disinclined to fly much between flowers. On balance I prefer to leave them be.

These hoverflies are much more energetic pollen stirrers ...

... and given the compatible flower types available at the moment, I'd better start keeping my eyes peeled for fertile seed pods.

Surely this will be the year!

Checking and Storing Oca Seed Tubers

A typical Oca harvest is a bit like a '60s Terry Thomas film -  sure to contain the odd rotter.
Oca tubers contain natural fungicides, and have good resistance to moulds and rots, but if they have been exposed to frost there is no escaping the fact that they are doomed to smelly putrefaction (have a look), and while most of the obvious casualties will have been spotted early when the tubers were lifted and washed, there are usually a few with minor damage that sneak through.
So now, a week or two after harvest, it's time to recheck stocks for any smelly surprises before storing them away until Spring.

And yes, several tubers (those on the plate) needed to be binned. If I had been desperate, I could have sliced off the affected parts, dipped the cut surface in wood ash, and the remaining material would probably have been fine for storing and replanting, but I'm in the fortunate position of having enough tubers.

As I sorted through, I was also carefully checking for any colour mutations, and when I saw the tuber below, just for a second I thought I had the potential to propagate a piebald strain.

Superficially, it looked like the area around one of the eyes had mutated to have black skin -  similar to the black Oca I obtained in the autumn.

But then I spotted an entry-hole leading to this tunnel. I think it's probably wireworm damage. I did notice one making a swim for it when I was washing the tubers.




With all the baddies removed, It's just a matter of selecting the good sized tubers, and putting them into storage. I use egg boxes (with lids closed to reduce drying out) as it helps to keep things organised, and makes occasional inspection easy...

...but the tubers seem to remain viable no matter how badly they are stored over winter; if left uncovered in a  heated room, they will shrivel up, but they will still sprout when the time is right.
If you are fussy about maintaining the visual appearance of the tubers, then aim to reduce the rate of moisture loss, for example by wrapping in newspaper, or a paper bag, or covering with almost-dry sand or sawdust, and placing them in an unheated room, or frost-free shed.
Be aware that if you go for the shed option, mice can develop a taste for them.

I'll start checking for shoots sprouting around late March.

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!

2009 Crop - Harvest Results

31st December. It's 25 days since the first light frost, and 16 days since harder frost completely killed the foliage.

Having made the decision that the crop is ready to harvest, and having lifted the first plant, two things immediately sprang to mind. Firstly, it's a heavy crop, definitely more productive than last year. And secondly, a nagging doubt that the stems are not all as dead as they could be, and consequently the plants could still be transporting sap to the tubers.

It's possible to go too far with the waiting game, but you don't know where 'too far' is until you go there, so rather than lift all the plants now, I've decided to lift them individually at intervals of a day or two, weighing each plant's tubers, to see if there is any increase in the yield-per-plant over the next couple of weeks. If I'm right, it should plateau-off at some point, and this will give an indication of the optimum harvest time.

Weighing the tubers from the first plant was a pleasant surprise. 795g of large, and 405g of small tubers, giving a total of 1200g. (Definition of large and small tubers here). Annoyingly, I did not weigh last year's harvest, but I would estimate this year's to be almost double.

Update: 28/1/10 Results of the waiting game:
It started off so well, the crop from the first three plants supporting my hunch completely, but then the weather put a spanner in the works by freezing the ground for a couple of weeks. This caused the local wildlife to get extra hungry, some of whom developed a taste for Oca tubers. The result of this was an unknown quantity of tubers being scratched up and carried away, and the experiment was ruined.

Anyway, here are the limited results of the experiment, which allow some conclusions to be drawn:

Plant 1 (16 days after killing frost) 1200g  (OPI= 0.99)

Plant 2 (17 days after killing frost) 1217g  (OPI= 1.01)

Plant 3 (19 days after killing frost) 1407g  (OPI= 1.14)

Then came a period of snow and frozen conditions when no plants were lifted until:

Plant 4 (26 days after killing frost) 1155g  (OPI= 0.98)

More freezing weather with starving crows, rats, and feral ring-necked parakeets helping themselves:

Plant 5 (34 days after killing frost) 717g  (OPI=0.62)

finally, abandoning the experiment, I lifted all remaining plants. Weights include damaged tubers:

Plants 6 to 12 (43 days after killing frost) average yield 1038g (OPI not calculated)

Tubers exposed and damaged by birds.

Conclusions

• Tuber weight may reach maximum at about 20 days after killing frost, or possibly even later. However the sample size here is so small that the data is not statistically significant, and no definite plateau was identified. It is quite possible that different temperature conditions would produce a different result.
• Average yields of over 1kg, and peak yields approaching 1.5kg per plant are achievable in the south of the UK without using fertilisers, protected cropping, or labour-intensive cultural methods, even when bi-cropped.
• Even if buried, tubers can be damaged by penetrating frosts, or raided by hungry birds, which can nullify any benefit of waiting for the purported optimum harvest time. Where freezing conditions are expected, it may be worth the extra work to earth-up, or provide other physical protection.  
• The primary bi-crop (tomatoes) did not show any noticeable variation in yield relative to the control bed (which had the same spacing, but with French marigolds instead of Oca). N.B. the tomato yield was not weighed, so this conclusion is based on subjective judgement.