Proper diagnosis of plant problems is a key factor in plant health management. For the next four months, AECI Plant Health will help Food For Mzansi readers to understand why their plants get sick, and how to manage better manage it. E-mail your questions and pictures to info@foodformzansi.co.za.
Thabiso Mchunu from Phuthaditjhaba in the Free State writes: I’ve really been struggling with a fungus on my potato plants. It started as tiny brown spots on the lower leaves, but now it’s all over the surface of the leaves. It even turned yellow and, long story short, my plants look really bad.
Dr André Cilliers, marketing manager at AECI Plant Health, writes: Howzit, Thabiso. What you’re struggling with is actually a well-known potato disease called “early blight”. The experts tell us that early rust of potatoes is caused Alternaria solani, a fungal pathogen that affects all parts of the potato plant, including leaves, tubers, and stems.
It is actually found wherever potatoes are grown and can be devastating if it is not controlled effectively.
Causes and infection
Any fungal pathogen has to penetrate the plant tissue in order to cause infection. Physical damage to the plant always serves as an easy point of infection for a pathogen. The same is true of early blight of potatoes.
Care should therefore be taken to prevent physical damage to the plants as far as possible. A windstorm, for instance, can carry soil particles that can damage the plant, and fungal infection will follow. Remember that the spores of Alternaria solani are literally all over.
Insect damage is also a source of physical damage, which promotes infection. One of the most devastating insect pests of potato is leaf miner (scientifically known as Liriomyza huidobrensis). The adult leaf miner often penetrates the leaf surface many times before actually laying the egg on the leaf.
Each of these penetration sites is a potential point of entry for a fungal pathogen. Control of leaf miner is therefore just as important as control of the disease itself.
The lesion caused by Alternaria solani on the potato leaf is best described as a series of concentric rings, looking almost like an old-fashioned stove plate. See picture above where the lesion is brown in colour.
An active lesion (one where the fungus is still alive) has a yellow edge. A “dead” lesion does not have this and is completely brown. This is a way of determining whether the fungus has been controlled. In the case of older lesions, the leaf in the center of the lesion begins to fall away, leaving a hole in the leaf.
Naturally, the less green leaf surface that the plant has, the lower the photosynthesis of the plant is, meaning that it will not grow optimally. Effective control is therefore critical.
How to control early blight
AECI Plant Health has the strongest technical support team in the country to support Mzansi farmers. The products from AECI Plant Health prescribed to control early blight are Barrier 450 SC, Procymidone (dicarboximide), Zinc oxide, L7604), CungFu (Copper hydroxide, L7526), Ventum (Mancozeb (Dithiocarbamate), L8691) and Chloroflo (Chlorothalonil (chloronitrile), L5171).
Barrier is a translaminar systemic product, meaning that the active ingredients move from the top surface of the leaf, through the leaf to the bottom surface.
This gives excellent control. Barrier is sprayed about four times during the season (always consult the product label for detailed information). CungFu, Ventum and Chloroflo are contact fungicides, and are usually sprayed weekly.
By keeping the plant itself healthy, its own ability to fight off pests and plagues is increased. Alexin is a unique product from AECI Plant Health which contains salicylic acid. This product switches the plants own immunity system on, making it more capable of fighting off fungal infections, and other stress events. Alexin is always a good option in a potato programme.
As mentioned above, insects cause damage which leads to fungal infection. Insect control is therefore just as important as the control of the fungal pathogen itself. AECI Plant Health top products for the control of leaf miner such as Gibsan (Cartap hydrochloride, L7018), Suntap (Cartap hydrochloride, L5760) and Mecti (Abamectin, L7008). These should also be sprayed according to label recommendations.
Other strategies for the control of early blight are to plant potato varieties that are more resistant to the disease, use good quality seed tubers and generally avoid physical damage to plants.
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