Thursday, December 22, 2011

Phosphorous Deficiency in Grapevines

Phosphorous deficiency is an occasional problem for grapevines. Under natural conditions, almost all vines have a beneficial fungus, mycorrhizae, associated with their roots, which helps them to acquire P, but sometimes the soil itself prevents P uptake.

The distinct symptom of phosphorus deficiency is the appearance of discoloured, redden leaves.

Ben South has found this patch of vineyard over 'ironstone'. In acid soils P forms very insoluble compounds with iron, and it binds strongly to the surface of iron oxides.

Most soils used for viticulture in McLaren Vale have low native concentrations of phosphorus and therefore phosphorus fertiliser inputs are normally required. In this case the 'ironstone' has bound up all of the soluble phosphorous in the soil.

Phosphorus is taken up by vines from the soil water in its soluble phosphate form which would also be quickly bound up to the 'ironstone'.

Contact James Hook for more information.

Thursday, December 8, 2011

EL 32 Bunch Closure

Light Brown Apple Moth damage to grapevines before bunch closure.

Bunch closure (EL 32) in tight bunched varieties is a critical time in the lifecycle of botrytis. The greater the amount of ‘trash’ or damaged berries from light brown apple moth (above) at bunch closure the greater the risk of botrytis.

Botrytis first infects berries as latent or ‘unseen’ infection, during flowering when the flower cap falls. A second type of infection begins to develop as berries press together. Botrytis fungal threads (mycelia) begin to grow from fungus on dead and dying plant tissues (such as ‘trash’ flower debris or damaged berries). This growth from dead material infects berries after berry softening and progressively increases in wet or humid weather.

Alternatively varieties grown with an open bunch configuration develop less rot in wet conditions. They have better airflow and lower canopy humidity after rain. A good canopy structure is a key part in botrytis control. High quality Shiraz that is picked in early March has a low risk of developing a significant level of botrytis before it is picked - without chemical application!