Necrotic Leaf Blotch A disorder of Pacific Rose. Prepared by Ross Wilson AgFirst April 2016

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1 Necrotic Leaf Blotch A disorder of Pacific Rose Prepared by Ross Wilson AgFirst April 2016

2 Introduction Pacific Rose suffer Necrotic Leaf Blotch (NLB) badly. The key is to minimise NLB is to: A. Avoid over cropping B. Avoid any stress, C. Keep a good level of Zinc in the tree. Necrotic Leaf Blotch is a disorder associated with several apple varieties. Golden Delicious suffers a mild level of NLB, Pacific Rose suffers NLB severely. NLB is aggravated by high crop load, periods of cloud and rain, low leaf Zinc levels and stress to the plant. It s basically a physiological breakdown. We think that cloudy wet conditions slow carbohydrate production and supply to the tree leading to the NLB leaf breakdown. Saprophytic fungi can colonize the decaying NLB leaf tissue eg Alternaria however they are not the causal organism. NLB is caused by an interaction of the microclimate and the plant. It is possible that even with the best practice management, P Rose will suffer unsustainable levels of NLB in some microclimates. Crop Load Management Crop load management is critical. In NZ on semi vigorous rootstocks eg MM106, we would aim for 6-7 fruit per cm2 trunk cross sectional area (TCA) on a young tree and 3-4 fruit per cm2 of branch cross sectional area (BCA) on a mature tree. On dwarfing rootstocks this could be increased to 8-10/cm2 TCA and 4-5/cm2 BCA. Cropping levels above these guidelines will undoubtedly result in higher levels of NLB. Avoid any stress Good water and nutrition management is also critical. The goal of the grower should be to never let a P Rose go into any stress. Monitor soil moisture closely and ensure the plant has good balanced levels of all

3 nutrients. Keeping good Zinc levels in the plant is also critical to minimizing NLB. My first recommendation is to use a Zinc based fungicide program. In NZ we use Metiram over bloom, and 2-3 Zirams post bloom. Both of these fungicides contain Zinc. We also make strategic use of foliar Zinc fertilizers post bloom when not using Ziram. Metiram is a soft black spot fungicide that contains Zinc. How much of the zinc in Metiram is plant available is debatable, however because P. Rose is russet sensitive we believe it is a good fungicide option over bloom. The zinc is a bonus. In NZ, the product label rate is 150 gms/100 litres water sprayed dilute to runoff (3kg/mature canopy ha). The Metriam product we use contains 700g/kg metiram active ingredient. Post bloom it is common to use up to 3 Ziram fungicides for blackspot and summer rot control. Ziram also contains Zinc. A survey carried out by Agfirst in 2001 showed a clear relationship between Ziram use and NLB. We apply 150 gms per 100 litres dilute (3kg / mature ha) of a product with 760g/kg Ziram. Post bloom and outside the russet sensitive period, foliar Zinc containing fertilizers are also applied. Typically these are applied at litre/ha and contain 70% zinc sulphate in a chelated form. Another good time to apply Zinc is during mid dormancy where Zinc Sulphate can be applied at kg/ha to a mature canopy. This would only be used in NZ under severe deficiency cases and prior to mid dormancy. Frequently asked questions (FAQ) What is the recommended analysis programme for establishing Zn levels within Pacific Rose in any one season? In NZ we measure Zinc levels with a leaf test. I would recommend a test in mid Nov and again early February in NZ.

4 The standard apple recommendation is that Zinc should be between mg/kg leaf tissue. My experience is that for P Rose you should be aiming for mg/kg if the sample is not acid washed and mg/kg if acid washed. Leaf testing should ideally include acid washing prior to the test to remove the surface zinc deposits that are not contributing to plant nutrition. The graph below (cut out of a research report) shows leaf zinc levels over a season using an acid wash. Note the average of Pacific Rose through the first half of the season was 50mg/kg. In the second half of the growing season it lifted to 100 mg/kg due to widespread foliar Zinc applications. The dark black lines are standard recommendations in the South Tyrol for apples. The green shaded box is the current NZ standard recommendation for all apples. See below a graph of the leaf test results for my own P Rose blocks over the past few years. We have had a history of low NLB levels by district standards. These results were all without an acid wash.

5 What are the options in relation to Zn applications, apart from Zinc based fungicides? Foliar Zn post bloom, but how many applications can you make thru the growing season and what window can these application be made in? There are many times and methods of applying Zinc. If the grower has a real problem, a Zinc Sulphate application in the dormant period is very effective at getting good levels of Zn into the tree. Standard practice in the Hawkes Bay NZ is Use a Metiram based fungicide program over bloom

6 Use 2-3 Ziram fungicides post russet sensitive period Add Zn chelate to every cover spray post russet sensitive period if you are not using Ziram Add Zinc Sulphate to your post-harvest foliar spray program. Can this product be mixed with other products apart from Captan and / or applied by itself? Zinc chelates can be added to standard fungicide Anything else to be aware of in using Zinc? Zinc can cause russet so don t apply over the russet sensitive period and only use registered products that have been thoroughly tested in your own microclimate. Also ensure compatibility with other products that are being included in the tank mix. Weather conditions, timing of application etc. Apply in conditions that maximize leaf uptake. If you google "necrotic leaf blotch in apples", you will also find several informative articles.