ANNE’S MAS­TER­CLASS

How can you stop white rot? Anne of­fers sage ad­vice

Amateur Gardening - - Contents -

QA grow­ing num­ber of gar­den­ers I know are hav­ing trou­ble with white rot that has stopped them from grow­ing onions and gar­lic. Are there any steps I can take to avoid this dis­ease, and does wa­ter­ing onions dur­ing dry spells in­crease the risk? Sheila Macre­ith, Salisbury, Wilt­shire

AOnion white rot is in­deed a hor­ri­ble and de­press­ing dis­ease of the onion tribe. Once the scle­ro­tia or rest­ing spores are in the ground, they can per­sist for 15 years or more, even if no onions are grown!

These dor­mant spores re­sem­ble tiny black poppy seeds and slum­ber on un­til plants from the onion tribe (Al­li­aceae) are de­tected. Al­li­ums give off vo­latile chem­i­cals and the spores re­spond by ger­mi­nat­ing to in­fect the roots. Whether from seeds or sets, onion fo­liage turns yel­low, the plant comes loose and, on in­spec­tion, roots have rot­ted and white fluffy fun­gal growth shows on the bulb base. Black rest­ing spores are pro­duced and fall into soil.

Since our gar­den be­came in­fected with white rot some 10 years ago, I’ve given up on onions, gar­lic and shal­lots, but we still en­joy good crops of leeks and chives. Or­na­men­tal al­li­ums don’t seem af­fected, ei­ther. Maybe their chem­i­cal sig­nal is weaker and fails to wake up the rest­ing spores.

Once in the soil, white rot is nigh on im­pos­si­ble to get rid of, so avoid­ing it in the first place is paramount. Num­ber one is to avoid putting par­ings from shop­bought onions into your com­post heap, as I have spot­ted white rot on the base of these. Be cau­tious about tools and the boots and shoes of vis­it­ing gar­den­ers as they may carry soil from in­fected sites. The feet of birds and an­i­mals, sadly, we can do noth­ing about.

White rot won’t ger­mi­nate when tem­per­a­tures are above 68°F (20°C), so a pat­tern of cooler, wet­ter sum­mers in re­cent years will have helped it to spread. I think it’s un­likely that wa­ter­ing onions en­cour­ages white rot, as it has to be present to start with. How­ever, wa­ter­ing dur­ing high tem­per­a­tures and hold­ing back dur­ing cool spells might be sen­si­ble, es­pe­cially on heav­ier soils.

Hav­ing wilted, loos­ened and top­pled in the ground, an in­fected onion shows signs of white fun­gal growth at the base As we have white rot in our soil, I’m sow­ing spring onions into a crate of clean com­post. Sow thinly and evenly, and they will not require thin­ning and will crop next spring

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