by Leslie Cox; Thursday; November 23, 2017

I wrote about crown rot in a previous blog…sometime last year. But I believe, because this fungus has such a horrible impact on the longevity our plants, it warrants bringing it front and centre again.

Crown rot a deadly disease which is caused by a number of different pathogen species in the Phytophthora genus. It is also known as Phytophthora rot, for obvious reasons. Once a plant is infected with this disease, it will succumb.

These types of pathogens, known as oomycetes, live in the soil and have great survival stamina…capable of lasting for long periods of time in the soil. In fact, almost indefinitely. When conditions are right, Phytophthora pathogens can infest many types of plants…annuals, perennials, vegetables, vines, shrubs, and trees. Almost none are immune to infection.

Wet conditions are a preference…like during the rain-leaden weather we typically receive in the fall in our region. If you should be unfortunate to have heavy, clay-based soil as we have here in this part of Black Creek, the poor drainage only increases the likelihood for crown rot infection.

Symptoms of crown rot vary amongst different plant species, but with all of them, there is almost nothing to be done once the pathogen takes hold.

One of the first indications a plant has crown rot will become evident when the weather heats up in spring. If the plant is sitting in a damp area of the garden…and your fall and /or winter has been a wet one, as our fall has been this year…its leaves may turn dull green or yellow in colour. Look closer and you will probably see the base of the leaf stem is a beige colour at the soil level. This is the Phytophthora pathogen attacking the plant tissue and its colour indicates it will have progressed to killing off that part of the plant.

Check your trees and shrubs too, as they are not immune. But symptoms are harder to spot, to be sure. It is usually young and / or smaller trees and shrubs which are affected…especially those which have been newly planted earlier in the season. Many gardeners forget the small root ball of a new tree or shrub often settles deeper in a freshly prepared planting hole. Sitting below the level of the soil could well be the death of the young plant.

A word to the wise. Make sure the crown of all plants, shrubs, and trees are raised up a little above the level of the surrounding soil when they are planted.

Okay…two words to the wise! Make sure you regularly check those new plantings to ensure they have not sunk down a little during ensuing watering sessions and rainfalls.

Of further biological interest…while Phytophthora species were once thought to be fungi, they have now been reclassified as oomycetes. Known as “water molds”, this is a group of several hundred organisms, within a number of different genera besides Phytophthora, which cause some of the most lethal plant diseases. Amongst the most notable are: sudden oak death, potato late blight (of Irish famine fame), and downy mildew in grapes. Other diseases these water molds can cause are: damping off, seedling blights, foliar blights, and root rots.

As mentioned earlier, oomycetes are capable of surviving for years in moist soil. They produce “resting” spores, called zoospores, which can wait almost indefinitely until conditions are suitable and a convenient host has been planted within striking range. But if the soil were to completely dry out, these spores are unlikely to survive longer than a few months.

Drainage is everything here…and some periods of drought might not be so bad after all.

 

Garden Fact:

Given favourable conditions, it only takes four to eight hours for Phytophthora species to infect a plant.