walnuts

Research Up-date

NZ Walnut Industry Group
 

One of the early findings from the blight control research is that Mankocide appears to be more effective at controlling the blight bacteria than the Kocide which most growers have used until now.

We asked Dr Tim Jenkins to contribute the following article to guide growers in the safe use of mankocide and provide information on any health or environmental risks.

In publishing this information on a 'new' treatment for blight, we note that the more familiar kocide copper spray is also considered a chemical that is dang-erous to both human health and the environment.

In considering the warnings Tim offers us in relation to mankocide we need to make sure that as growers we are following best practice in our handling of all toxic sprays, and carefully assessing impacts on health and the long-term sustainability of the environ-ment.

Mankocide: for control of walnut blight

Currently, most growers spray against walnut blight using kocide (active ingredient copper hydroxide). Recently it has been shown that mankocide (a combination of copper hydroxide and the fungicide/bactericide mancozeb) is more effective than kocide in reducing blight levels. Mankocide was trialled, along with a number of other sprays, in the NZWIG Sustainable Farming Fund walnut blight project. We found 55% less walnut blight on trees sprayed with mankocide, as compared to those sprayed with kocide.

The treatment included only 90% of the amount of copper used in the kocide treatment, but gave 2.3 times the level of control. This means that it is very likely that we can drop the rate of copper and still achieve at least the same level of control as with Kocide. That would be a very positive result in terms of environmental sustainability.

The main environmental concern in management of walnut blight is the potential accumulation of copper in orchard soils. We have also shown there is potential to use a lower rate of copper, while still achieving blight control, if we adopt reduced but strategic spray timings.

It should be noted that mankocide is not allowable in organic orchard systems, whereas kocide is allowed in restricted amounts. In Australian trials Bordeaux mixture was the most effective of the sprays acceptable under organic certification.

The mancozeb component of mankocide is a common fungicide used on a wide variety of crops. Its active ingredient is dithiocarbamate. Application to walnut trees is an "off-label" use but is allowable as long as the residue in the kernels is no greater than 0.1 ppm. We collected walnuts from trees sprayed fortnightly with mankocide throughout the 2003/2004 season, and sent the kernels away for testing at Hill Laboratories. There was no detectable residue of dithiocarbamates (detection level is 0.02 ppm, clearly below the permitted level).

The rate of application used so far in New Zealand trials is 205 g per 100 L (equivalent to 380 mL per 100 L), sprayed to runoff. Australian research has indicated that a higher rate (approximately twice the concentration) is more effective, probably due mainly to the amount of copper applied. This higher rate will also be tested in the upcoming New Zealand growing season. The increased effectiveness of mankocide over kocide is likely to be due to: (1) the toxic effect of mankocide on copper tolerant/resistance walnut blight bacteria and/or (2) one of the active ingredients makes the bacteria more susceptible to the other ingredient. We are therefore assessing the potential of ingredients other than mancozeb that might be used in combination with copper to enhance walnut blight control.

Potential health effects are generally considered minimal if appropriate precautions are taken. The related product maneb was found in a rat trial (in combination with paraquat) to cause damage to the brain in the same area as is affected in sufferers of Parkinson's disease. There is also a case of the development of Parkinson's disease in a man, seemingly related to prolonged, unprotected exposure to large amounts of maneb. Through animal trials, mancozeb is described as having low toxicity to mammals including humans, and is considered non-carcinogenic and to have no reproductive inhibition or mutagenic effect. The breakdown product ethylenethiourea (ETU) has been linked to reduced thyroid function and thyroid tumours in an animal study involving feeding with mancozeb. Some websites that discuss potential health effects include:

http://www.health-diets.net/research/parkinsons.htm
http://www.cnn.com/HEALTH/library/DS/00295.html
http://infoventures.com/e-hlth/pestcide/mancozeb.html

(The third of these is the most comprehensive and also lists potential environmental and other effects).

While pointing out potential health risks, it should be noted that dithiocarbamates are very common sprays worldwide and in New Zealand. As with most other sprays protective clothing, masks with effective respirators, goggles and gloves should be worn, and spray drift should be avoided, especially onto unprotected people. A recommendation on the third website is not to enter a treated area until 24 hours has expired unless wearing suitable gear to avoid skin contact. Note it is an irritant to eyes and skin and one source indicates it can cause permanent corneal (eye) damage in severe cases. Rinse thoroughly in the case of eye or skin contact (use soapy water for 15 minutes on skin). Seek medical attention.

To minimise environmental impacts, avoid spray drift and do not dispose of wash water into waterways as the mancozeb component is toxic to fish. Predatory mites (that can control plant pest mites) are also susceptible to mancozeb. More detail of potential environmental impacts and ways to address them are given in the third website.

In summary, mankocide provides significantly better control of walnut blight than does kocide, when applied at the same rate of copper per hectare. We will be working to reduce this rate while retaining blight control effectiveness, by strategic spray timing and other possibilities. Mankocide (the mancozeb component) has a wider range of potential health and toxicity effects as compared to kocide but the risks are minimal if normal precautions are taken while spraying.

Tim Jenkins