Plants need water to thrive so if you want them to grow extra well you should give them
extra water - right? Wrong. As it turns out, giving plants too much water is just as likely
to kill them as not watering at all. This is especially true when it comes to using
California native plants or California climate-appropriate plants. Why? Because they
were simply not meant to have a lot of summer water. After all, the hallmark of the
Mediterranean climate zone is wet winters and dry summers.
Your plants will seemingly look healthy one day and the next day the leaves will start to
droop. Time for more water? Not necessarily. If they are suffering from Phytophthora, a
soil-borne pathogen that infects trees, woody plants, and even vegetables, more water is
the last thing that they need. Because water, especially in the warm days of summer
when the pathogen can spread easily, is what’s causing them to die.
The name Phytophthora comes from Greek - φυτόν (phytón), "plant" and φθορά
(phthorá), “destruction”; and means "the plant-destroyer". It’s a genus of plant-damaging
oomycetes (water molds), and it’s a gardener’s worst nightmare. (One strain of
Phytophthora, known as Phytophthora Infestans, was the fungal agent responsible for
the Great Irish Potato Famine.)
Phytophthora attacks the roots of a plant or tree, making them unable to absorb
nutrients or water because of the dead root tissue. Preventing the spread of
Phytophthora is difficult, but possible. Since the mold thrives in water, the most
important aspect of prevent is correct water management. Water should be uniformly
distributed to each plant, without causing the plant to stay wet for unnecessary amounts
of time and your irrigation system should be moving the water away from roots.
Sometimes the results can happen almost overnight; sometimes it takes a while,
causing weakness and a slow collapse in the affected plants over a course of months or
years.
Poorly draining soil can exacerbate the problem because phytophthora only needs four
hours of standing water to germinate. In clay or compacted soils, it helps to plant on
mounds 8 – 10 inches tall. But phytophthora occurs in all types of soils. In fact, I
encountered it in a garden with very sandy soil.
Although there is no cure for Phytophthora, there is a way to prevent and treat it. I spoke
to Jerry Turney, Los Angeles County Plant Pathologist to learn more about it.
Unfortunately, plants that are already stricken need to be removed quickly and disposed
of carefully so that you don’t spread the pathogen. But what about the plants that are
left that are showing signs of failing?
Jerry’s advice was to apply a solution called Phosgard. It is available from Leaf Tech 1-
800-350-5323. Here’s how to apply it:
1. Move your mulch away from the base of the tree or plant when applying so that
the solution will go straight into the soil around the plant. (Replace when
afterward but remember, as always, to keep your mulch layer away from the
base of your plants and trees.)
2. Dilute the Phosgard solution at a rate of 1 tbsp. per gallon (1/2 oz) and soak the
soil around the affected plants weekly during summer and monthly through the
winter. If there is a lot of precipitation, reduce the frequency because you don’t
want your soil too saturated - wet soils encourage phytophthora root rot.
3. Once the plants show some good growth you can reduce the frequency to every
three months. Watering can be reduced to once a month in spring and every two
weeks (if necessary) in summer.
4. You can also spray the plant foliage with the diluted Phosgard solution.
5. Mulch twice a year and add gypsum to the soil (gypsum should go underneath
the mulch).
6. He also suggests using mycorrhizae on the root ball of when planting.