2017 has been a particularly bad year for cavity spot and damping off disease (both caused by Pythium species) in carrot and parsnip crops. Now trials have shown that biorational nematicide Nemguard DE, can provide useful additional suppression of Pythium spp. when used for the suppression of nematodes due to the action of the allyl-polysulfides contained within the garlic extract active ingredient on fungal pathogens.
Ecospray Research Director Dr Murree Groom, who was the first person to show that Pythium violae was the fungus behind cavity spot back in 1985, explains: “Cavity spot has come back with a vengeance in 2017. Pythium species attack many seedlings causing damping off and reducing growth in both carrots and parsnips during the first few weeks after drilling. This not only affects plant stand but can also lead to cavity spot.
“The impact of metalaxyl-M based fungicides and metalaxyl-M based seed treatments appear to have reduced in recent years and we have also seen also evidence in our field trials of a compounding effect from oxamyl based nematicide treatments. Nemguard treated carrots seemed to suffer less Cavity Spot than either the untreated control or those treated with oxamyl,” says Dr Groom.
“The main protection against damping off in carrots and parsnips are seed treatments containing metalaxyl-M. However, under certain conditions we can see some damping off occurring even with treated seed,” agrees Agronomist Howard Hinds of Root Crop Consultancy Limited.
“Cavity spot is one of the carrot and parsnip industry’s biggest challenges. This year has been particularly bad following a mild and wet September and October. Soil treatment with metalaxyl-M gives some protection but as it is applied soon after emergence of the crop, it often runs out of steam by the autumn. We are constantly looking for products to give added protection,” says Howard.
Trials results presented in a poster at this year’s Onion & Carrot Conference have shown significant gains in plant stand and reductions in Cavity Spot where Nemguard was applied at drilling, and laboratory studies have also shown an inhibition effect of the active ingredient on two Pythium isolates, one of which was obtained from a cavity spot lesion from Yorkshire in 2017.
So what is going on? The allyl-polysulfides contained within Nemguard DE combine in the cell to deplete ‘low molecular weight thiols (LMTs)’ and the cell effectively loses its capacity to control this reaction and dies. This process can be described as oxidative stress; releasing a cascade of reactive oxygen molecules which react with other parts of the cell, rapidly closing it down. LMTs are found in all oxidative organisms, so fungi are affected in the same way as nematodes, bacteria and insects.
“There are at least four biochemical sites of action in the cell once the allyl-polysulfides have been taken up,” adds Murree, pointing out that this significantly reduces the likelihood of resistance developing. “The fungicidal effects have also been seen against another Fusarium species where the same active ingredient has been applied as a nematicide to turf.” As a result, Ecospray has already obtained an incidental fungicidal claim on the label of that product and is now in the process of applying for a similar claim for Nemguard DE for Pythium control on carrots and parsnips.
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