I live in North York zone 6, this is the second year I have had these Issai hardy Kiwi plants Actinidia Arguta in dry clay soil but planted following instructions with compost loam etc. they are in full sun, with a watering system. They started out well but now they have a strange white mark around the edge of the leaves 3 of the 4 plants are effected
Thank you for contacting the Toronto Master gardeners concerning your Kiwi.
In general Kiwi plants are dioecious, meaning that they require a male and female plant of the same species in order to set fruit. There are some self-fertile varieties (not requiring a male pollinator ) Issai being one of them. Even though this variety is self-pollinating it is still advisable to add a male vine of the same species for increase in fruit production.
Kiwi grows best in full sun (6-8 hrs.) and fertile, well-drained, sandy loam or clay loam soil with moderate water-holding capacity. It should be noted that Kiwi plants require a soil pH of 5.6 to 6.5. It is recommended that you take a soil test before planting Kiwi. We recommend the City of Toronto’s Guide for Soil Testing in Urban Gardens, which outlines a step-by-step process you can follow. Click here to read or print it. The University of Guelph’s Agriculture and Food Laboratory information can be found here.
If your soil test indicates that the soil pH is too low (too acidic) or too high (too basic), you can amend the soil to improve the pH. If your pH is too high you can amend the soil with the addition of elemental sulfur. Alternatively, if your pH is low, you can amend your soil with lime. Make sure to follow directions on the package. Soil amendments are readily available at your local garden center.
Kiwi plants do not like wet feet and will suffocate in soils that are waterlogged for more than a few days. This will make the plants more susceptible to disease. And root rot. The yearly addition of organic matter aids in soil aeration, drainage and the capacity to hold water and nutrients.
Now, on to to your question, what is the white deposit on the leaves? From all research, we cannot find any disease that shows the symptoms that you are seeing. From your photo, it does not appear to be powdery mildew. It appears to be a deposit of some sort. You mention that you have an irrigation system, do you have a water softener as well? Is the water softener connected to the water line that supplies your irrigation system? If so, it could be salt deposit that has dried up in the sun, or it could be a fertilizer deposit.
Our advice would be to monitor your plants and see if it affects the leaves further up the vine: those that do not have water splashing directly on them.
The following links give additional information on plant care:
For more information on the general care of Kiwi see the following links
https://catalog.extension.oregonstate.edu/em9322/html
For a complete list of Kiwi disease see this link
Good Luck with your Kiwi plants.