23 blocks of the Rua cultivar were evaluated for symptoms associated with multishoot disorder over 6 Canterbury farms. Symptoms were present on 7 blocks on three farms, while other Rua blocks were performing normally.
Rua is the most prevalent cultivar with symptoms of Multishoot Disorder, previously known as “Bluebottle syndrome,” though other cultivars can be affected.
A severity rating scale was developed, and a novel symptom described—wavy vascular tissue with internal brown flecking
Virus/phytoplasma/liberibacter causes were ruled out by MPI’s Plant Health and Environment Laboratory using a new screening technique called “high throughput testing.”
Multishoot disorder is not a permanent condition—a block can “snap out” of the problem through cut-down or other (unidentified) stimulus.
11 leaf nutrient tests along with soil tests were examined by Murray Craighead, Nutrient Solutions Ltd, looking for evidence of nutrient deficiencies, particularly Zn and Boron. His recommendations:
Take leaf tests earlier (late Oct) than the standard recommendation (early Dec) to identify short-lived nutrient limitations due to plant uptake and/or distribution
Apply a micronutrient blend containing B and Zn, with a wetter, in March and again during bloom
Make sure soil pH is not above 6.3, and other nutrients (such as P and Mg) are not excessive
The role of nutrient deficiencies and compounding factors such as heavy or compacted soil is not clear yet
Ongoing management of Rua is important as the variety represents over 20% of the harvested crop ($2.4 million)
Recommendations
We suggest trialling the possible management approaches:
Pruning:
Hand prune a few old canes per bush
Cut down
Heavy side prune
Nutrients:
Foliar micronutrients at bloom (“Bud Builder” or “Bud Wiser”), applied with organosilicon wetter
Check of soil health (visual assessment)
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