All set to sow seeds in the fight against onion blight

Onion blight is a canny operator, hiding in the soil and waiting, for years if need be, for allium sets to arrive

WHEN it comes to pesky and persistent garden pests, it would be fair to say that OPWs walled kitchen garden in the Phoenix Park has had more than its share.

Over the last number of years, and with differing degrees of success, its gardeners Meeda Downey and Brian Quinn have battled everything from gooseberry sawfly to sap-sucking aphids, root-eating cutworms and marauding badgers. And thats not to mention the vine weevils, slugs, leather jackets, carrot fly larvae, stray rabbits, acquisitive squirrels and covetous crows that they’ve also, at some stage or other, had to contend with.

Meanwhile, as regards potentially damaging plant diseases (the even uglier sister of garden pests), the two gardeners have successfully treated powdery mildew on their pumpkins and staunchly fought off the threat of potato blight. But despite all that, even they now admit to being stumped by the dreaded onion white rot.

Last summer (despite careful crop rotation), many of the allium crops in the OPW’s walled garden succumbed. to some degree, to this persistent fungal disease, which hit the crop for the second year in a row and with fairly destructive results. Half of the spring onions collapsed – their foliage yellowed and their decaying roots were covered in the white fluffy mould that is the signature of this soil-based fungal disease. Most of the rest of the main onion crop also showed obvious signs of infection, as did the leeks, with the singular exception of the red onions.

As to how the disease first came into the garden, the gardeners admit with frustration that they’ll never really know. Brian suspects infected onion sets but Teagasc advisor Stephen Kildea thinks this is probably unlikely. “You will occasionally see other onion diseases being brought in accidentally with sets, but I wouldn’t think that onion white rot is likely to be one of them.

“But the disease is not uncommon in some parts of north County Dublin – I’ve seen it in the fields of commercial growers, in allotments and back gardens. Sometimes people might get a gift of a plant from a neighbour and it will come in on the soil.”

The OPW gardeners first spotted the symptoms two years ago. “We noticed it for the first time in the summer of 2009 (the cool, damp summer provided ideal growing conditions for the disease) and took a gamble on preventing it spreading by practising strict crop rotation and careful garden hygiene. But, to be honest, it’s difficult to prevent it from spreading once it’s got a hold,” says Brian.

Once it has infected a crop (which can be any member of the allium family), the onion white rot fungus manages to survive in the soil for years or even decades by producing tiny black “sclerotia” that are the size of poppy seeds.

These are initially produced on the roots of infected plants, before falling off into the soil where they remain dormant but viable for many years. The trigger for germination, when it finally occurs, is very simple – just the very nearby presence (1cm away) of any member of the allium family, or more specifically its roots, which excrete the required mix of volatile chemicals.

Onion rot, so it seems, is the perfectly designed pathogen, the ultimate horticultural time-bomb, stealthily biding its time until the host plant is present. Ouch.

As to exactly how long it will bide its time, that is a matter of debate – the RHS, in its book Pests and Diseases, says onion rot (or Sclerotium cepivorumto give it its Latin title) is a serious disease that can persist in the ground "for more than seven years" while others put it at over 20.

What’s worse, there is no easy cure. “Remove and burn infected plants as soon as they are noticed and do not grow onions or related plants on the same site for at least eight years,” states the RHS guide grimly.

So what are the OPW gardeners to do? In an attempt both to continue growing allium crops and avoid the introduction of any other damaging onion diseases, they’ve decided to avoid using sets entirely and are instead sowing their onions and shallots from seed.

“Sets are much easier and quicker to mature but the benefit of seed is that we can at least be sure that the crops begin their life in the walled garden disease-free,” says Brian (he’s not so certain, however, that they’ll stay that way).

And so last week he and Meeda busied themselves in the nearby OPW Klondyke gently heated glasshouses (to 15 degrees), where they sowed seed of Red Baron, Ailsa Craig and Banana into 100-cell modular seed trays.

Using their fingers to gently press the good quality, damp seed compost into each individual cell, the gardeners sowed one seed to each cell, before “riddling” or sieving a thin layer of compost over the tray, lightly watering it, labelling it and then covering it with a sheet of glass.

If all goes according to plan, the baby onion and shallot seedlings will begin to appear in a week to 10 days. Then, while still at what vegetable expert and author Joy Larcom describes as the “crookneck stage” (where the fragile seedlings are still bent over), they will be potted on into larger modules before being hardened off and planted out in the walled garden later in spring.

Meanwhile now, in early spring, as gardeners everywhere order their seed potatoes, is the time to consider the perennial threat of another depressingly damaging fungal disease – Phytophthora infestansor potato blight. It seems that despite Brian and Meeda's reservations as regards the tastiness of the famously blight resistant Sarpó Mira, they've finally been persuaded to try growing the variety this summer in the walled garden.

And so, along with other earlyish potato varieties such as Colleen and Orla, main-croppers Arran Victory and Records, the pink seed potatoes of Sarpó now lie chitting in neatly labelled trays in the OPW glasshouse.

“We’ll still be taking a bit of a gamble with Arran Victory and Records, but by planting mainly earlies and Sarpó, we’re hoping, fingers crossed, to escape the worst risk of blight damage.”

So forget the knock-out punch when it comes to the never-ending battle against nasty fungal diseases in the garden. Instead it seems the OPW gardeners have decided, for the moment at least, that their best tactic is the classic “duck-and-weave”.

The RHSI’s Wildlife Friendly Gardening Seminar takes place on Saturday March 5th at the National Botanic Gardens. Glasnevin (cost is €65 including tea/coffee and lunch). Guest speakers include Dermot O’Neill, Eanna Ni Lamhna and Niall Hatch of BirdWatch Ireland. rhsi.ie/ 01-235 3912

The OPW’s Victorian walled kitchen garden is in the grounds of the Phoenix Park Visitor Centre, beside the Phoenix Park Café and Ashtown Castle. The gardens are open daily from 10am to 4pm

Next week Urban Farmer will cover preparing and planting up a new strawberry bed in the walled garden

Fionnuala Fallon is a garden designer and writer

WHAT TO: sow, plant and do now

BEGIN TO SOW:

In modules under cover, for planting out later in the vegetable garden: asparagus, broad beans, globe artichokes, beetroot, Swiss chard, early Brussels sprouts, summer cabbages, cauliflowers, red cabbage, carrots, parsley, chives, Welsh onions, spring onions, leeks, bulb onions, shallots, parsnips, early peas such as Kelvedon wonder, radishes, kohl rabi, white turnips, landcress, rocket, mizuna and salad leaf mixes. Also sow hardy annuals such as limnanthes, calendulas, convulvulus tricolour, and so on. These will flower early and attract insects to help with pest control. Remember to keep seedlings frost free once they’ve germinated, but don’t grow them in too much warmth or they will be too soft.

UNDER COVER IN A HEATED PROPAGATOR:early tomatoes, aubergines, calabrese, celery, French beans, sweet peppers and chilli peppers, to grow on using a heat mat before hardening off and then transplanting on into polytunnel in late spring.

IN A POLYTUNNEL(either direct-sow or sow into modules, using fleece at night for frost-protection): broad beans, carrots, Ragged Jack kale, ruby chard, mangetout and early peas. (Sowing details courtesy of Nicky Kyle, nickykylegardening.com)

PLANT: very early, chitted potatoes into 2L pots indoors, to move to cool, frost-free spot before planting in polytunnel using fleece at night for frost-protection; garlic (in individual cloves), direct outdoors or into modules to transplant later; Jerusalem artichokes.

DO :plan this year's vegetable plot; chit potatoes; order seed (catalogues), weed polytunnel crops, sort through stored vegetables and discard any that are rotten or frost damaged.