Most of the major onion diseases are caused by fungi or bacteria, or sometimes by soil nematodes. As organic gardeners, though, it will not really help us much to know which disease we’re dealing with.
This is because once our plants become sick, organic disease control measures are very limited, regardless of which specific disease is making our plants sick.
The good news though, is that most onion diseases can be prevented.
There are five easy things we can do to prevent diseases from attacking our plants in the first place:
1) Water at ground level. Onions should be watered at ground level, either by a dripline, leaky pipe, or irrigation tape. Keep a careful eye on soil moisture levels to assure that the soil stays moist enough to remain loose, crumbly and easily pushed aside by the onion once it starts forming its bulb. But make sure also that the onions do not stay wet.
2) Be careful with mulch. A light, airy mulch like straw can help keep onions from drying out too much between waterings, as well as reduce weeds. However, if you do mulch them, make sure you have well-drained soil, and wait until the plants are well-established and strong before applying mulch.
3) Provide good air circulation. Space plants no closer than 4” apart, and don’t hem them in with taller or denser plants that will block or inhibit air movement.
4) Spray actively aerated compost tea. Properly prepared compost tea can be sprayed on plants to give them a huge boost in disease resistance. I’m not speaking of ordinary compost tea, but actively aerated compost tea, which contains billions of beneficial fungi, bacteria and other microorganisms which outcompete the pathological disease-causing organisms.
5) Practice crop rotation. This is an important practice throughout the garden, and requires careful planning and good record-keeping. When you draw up your vegetable garden layout at the beginning of the season, make sure you don’t plant crops in the same place as the previous year. Crop rotation is more challenging when you have a smaller garden, but do the best you can.
Some fungal and bacterial diseases may be controlled by spraying with neem oil. However, my personal preference is to immediately remove diseased plants from my garden when I discover them, to prevent their spread as well as carryover to future years.
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