Soil pH – the cure for clubroot in cabbages

Clubroot really is a revolting disease -that’s literally the reaction of any farmer who pulls out a sickly-looking plant and sees what the root system looks like. Clubroot is also destructive enough to wipe out a crop.

Luckily, it usually spreads gradually, so it can be noticed and stopped before that point. Occasionally, however, it starts as a small patch which may elude the farmer. Labourers working in the land may spread the spores to many areas, and if climatic conditions are favourable, it can fast become destructive.

All cruciferous crops are susceptible to varying degrees. Many other genera can also host the disease without showing the conspicuous root swellings. These include red clover, perennial ryegrass and even weeds such as Polygonum. Canola is also a host crop but shows only mild symptoms.

If you plant these crops on a land with a small or even barely noticeable infection, you can get a huge surprise when the land is later planted to cabbages.Although the disease is mostly spread in soil and mud adhering to shoes and implements, it’s also spread in water. I’ve seen a rainstorm wash soil from an infected farm into a river. The lands of farmers pumping water from the river downstream became infected.

The fungus is very persistent and can remain in the soil for up to 10 years without a host crop. It can only multiply in a host crop’s living cells and is more active in acid soils. Although it favours warmth, it can be active in a wide temperature range. The ideal conditions are warm, wet conditions in an acid soil.

Recognition and elimination
The first symptoms are the plant starting to wilt despite adequate soil moisture, and yellowing, as the root system can’t extract moisture and nutrients from the soil properly. The disease always starts off in patches which, since it’s more active in wet conditions, farmers sometimes attribute to damage from excess water after heavy rains.

There’s a real danger when the farmer assumes and doesn’t physically check, as the disease will be substantially more severe at the next planting.Fungicides and soil fumigants registered for control may reduce the spread of the disease to a fair extent, but they’re expensive and ineffective over the long run. The only practical treatment is to recognise the fungus’s weakness – its spores can’t germinate in alkaline conditions. The disease won’t develop when soil pH is about 7.2 to 7.3 (this is the actual soil pH or water pH and not the KCL pH).

It is worthwhile withdrawing an infected land from cruciferous crops until the pH has stabilised, which may take up to three years. Determine how much lime would be needed to do this according to your soil analysis by consulting your fertiliser adviser. Many growers would consider such a long withdrawal too great a sacrifice. In this case, use a percentage of hydrated lime, which is about 30 times more active than agricultural lime. Usually you’ll use 1t of hydrated lime and make up the balance with agricultural lime. Hydrated lime is much more expensive, but usually worth it for vegetables.

I’ve seen many vegetable farmers cure their clubroot problem by liming. You have to increase the soil pH above the ideal level, but a pH of 7.3 is no limitation, but clubroot is.The higher pH will certainly be suitable for most vegetables, but perhaps be a problem for scab on potatoes, so confine potatoes to scab-resistant varieties. 

Bill Kerr (016 366 0616 or e-mail [email protected]).