Thursday, August 30, 2012

Stroby Resistance - UPDATE 30/8/2012

Powdery mildew strains resistant to the strobilurin fungicides (Group 11 fungicides - Amistar, Cabrio and Flint) have  been confirmed in vineyards in South Australia, Victoria and Western Australia in the last 12 months. Testing of samples of powdery mildew in these vineyards confirmed a high level of the resistant gene – which prevents  the strobilurin fungicide being effective. 

Flint is a group 11 fungicide.
The development of these recent strains signal the need for a different approach to using these products on  grapes in Australia. 

News about stroby resistance in Australia has been slow to be made available to the grape and wine industry. There have been changes to the resistance management guidelines but is this enough to raise awareness? We think this is one of the most significant developments in the last ten years. Do you know about it?

In Europe and parts of the USA where strobilurins have been used for a longer time, resistant strains of both powdery mildew and downy mildew have been causing crop losses in vineyards since the early 2000s. To make matters worse once stroby resistance develops it is permanent.

Grape growers need to ask question about stroby resistance and how it might affect their disease control. If the wine industry waits to be lead by chemical distributors and resellers - the spread of information will be very slow.

Monday, August 27, 2012

Soil Moisture Levels - UPDATE 27/8/2012

Heavy clay - 17/8/2012. Click on the graph to see larger version.
Topped up by heavy rain during August, soil moisture levels are very high around McLaren Vale. Most of DJ's Growers 100x SENTEK Diviner 2000 sites are showing all time high soil moisture. For these sites it is the wettest it has been since they were installed in 2007.

The high levels of soil moisture can be seen by the graphs reaching its highest value on the right of each reading.

Red clay - the soil moisture level is significantly above the last two winters.

Thursday, August 9, 2012

Olive Nutrition - Q&A

DJ's agronomist Sam Freeman looks at why olive trees turn yellow in winter.

Olive foliage is naturally green, persisting for about two years on the branches in all seasons. There are two main reasons that an olive tree may have yellow leaves at this time of the season.

Olive grove on light, sandy soil - 5/8/2012.
Yellow foliage on an olive tree - 5/8/2012.
If there are just a few yellow leaves scattered throughout the canopy, it may be natural senescence. Since olive trees are not deciduous, they have continual leaf loss through the season. Most olive leaves have a lifespan of two to three years, at which point they will yellow and fall off naturally.

If the majority of the canopy is yellow, it may be a nutrient deficiency that is causing the yellow leaves. Olives do not need a particularly nutrient rich soil, but yellow leaves is an indicator of low plant nutrient levels. A little additional fertiliser once a year early in late winter or spring does improve plant nutrient status, leaf colour and boost the production of your olive trees.

If the yellowing is not caused by nutritional problems then consider if waterlogged soil is the problem. 
Olive with natural grey/green leaf colour.
Overly wet soils may cause olive trees to become stressed and have yellow leaves when they are unhealthy. Poorly draining soils also leads to fungal diseases. When a fungus initially infests a tree, leaves prematurely yellow, die and drop off, making the once healthy olive tree look nude or sickly.

Thursday, August 2, 2012

Olives - Sooty Mould and Vine Scale - UPDATE 4/8/2012

Monitoring for Olive Black Scale (Saissetia oleae).
At this stage of the season black olive scale can be seen on olive trees. The adult females are very easy to recognise on the olive tree stems. They are dome shaped dark black in colour and 2-5mm in size. Note also the co-responding sooty mould. 

During spring use copper fungicides to reduce the amount of sooty mould.

At this point of the season scale are dormant underneath their protective shell where they lay egg.

During the December and January cream coloured 'crawlers' hatch from these eggs and move up the stems. They usually settle along the veins of  young leaves. At this stage they don't have the impervious shell of the adult and can usually be killed with one or two applications of insecticidal oil about two weeks apart. It puts an oil film over the young 'crawler' and suffocates it. 

Spray oil during December to stop the crawlers.

If the crawlers are allowed to live, they will moult after about one month and then migrate to the young stems and twigs of the tree. Here they will mature and lay more eggs and their protective brown shells will be impervious to sprays.

Olive black scale. Squash the scale between your fingers to see if it is alive. If it is alive, then your fingers will be wet from the juices squeezed out. If it is dead  then your fingers will be dry and dusty.

Olive black scale and co-responding sooty mould.

Bad infestations of live mature scale may need spraying with an insecticide.

As the scale feeds they excrete is a sweet, sticky, 'honeydew'. A  fungus known as sooty mould feeds on this food and multiplies until the entire tree may be covered with the black sooty mould.

The leaves are coated with the black deposit so the sun's light can't  penetrate the leaves properly. Therefore photosynthesis can't take place efficiently. This results in a stunted  and unhealthy tree with poor crops.

To make the problem worse, sweet 'honeydew' on the leaves also attracts large  numbers of ants. Ants constantly move over the scale, they frighten away the small wasp parasites which in normal cases would  keep the scale under control. 

The good news is that healthy olive trees don't get the scale, sooty mould and ant infestation to any great extent. More good news is that heavily infested  trees are easily fixed.