Archive for March, 2012

The longer you have a garden, whether it be vegetable or ornamental, the more likely you are to be slowly building up a reservoir of problems. There are several viruses that can take hold and be real problems in the home garden. Tomato spotted wilt and tobacco mosaic virus, for example. There are some other very serious viruses (tomato yellow leaf curl virus) that are not yet in WA. Then there are other problems such as phytoplasma that cause witches brooming (not to be confused with witches brooming from boron deficiency). And then there are root rots and other soil borne diseases such as Sclerotinia and nematodes.

All of these diseases, once contracted by a plant, usually kill it. In low numbers, nematodes may simply stunt plants and largely go unnoticed. Often plants succumb to other problems which are often secondary to the real issue of nematodes. Stem cankers in roses are often the result of undiagnosed nematode problems.

Chemical control of these problems is often only achieved with highly toxic chemicals. Sclerotinia forms highly resistant resting stages that last for years in the soil. Viruses and phytoplasmas cannot be cured, one can only stop them spreading and that is why it is really important to pull out and get rid of infected plants.

Non-chemical measures such as hygiene and rotation become critical in the control of all these diseases. For viruses and phytoplasmas, control of the vectors of the diseases is the only way out. Depending on the individual disease, that is often a leaf hopper or thrips (though Tobacco Mosaic Virus is spread mechanically, not by insects). And the problem with these pests is that they can fly around and hop on and off plants very easily. This is where weed control and control of alternate hosts becomes important. Even capeweed is suspected as an alternate host of some viruses. Alternate hosts are not necessarily from the same family. They may even be other ornamental plants in your garden such as petunias.

So where you are sowing successive crops of tomatoes, eggplant or potatoes, for example, you can expect to have increasing levels of virus building up over the years – assuming your level of insect control is not perfect. Sometimes the only way around it is to have a fallow period where no hosts are present for a period of time – say three months. And of course one problem here is that because these little critters can fly, the alternate host may not be in your backyard! It could be on the front verge or in the neighbours garden.

For nematodes you may need to have a fallow period to get numbers down to low levels. If that is not possible try sowing a non-host for a while such as any grass or sweetcorn. Incidentally, the reason we grow roses on fortuniana ropotstock in Western Australia is because it is relatively resistant to nematodes, unlike many of the rootstocks used in the Eastern states. Further information on some tomato spotted wilt can be found here. This farmnote gives information on Tomato Yellow Leaf Curl Virus and this, on Tobacco Mosaic Virus.

Here is a really good chart, albeit for Victoria, on what to use in various rotations and why.

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