Botrytis or Grey Mold (B. cinerea) is the single most common fungus in Mother Nature’s arsenal. It is the primary attack weapon to attack weakened plants or fading flowers in order to start the recycling process. So when you look at the grey mould on your plants, understand that this is nature trying to recycle the nutrients in the plant to make them available for other plants; it is part of the natural cycle of things rather than a personal attack on your garden.
What does it look like?
The intial sign on flower petals is a small, light coloured spot (sometimes surrounded by a maroon halo).
Under conditions that fungus thrive in (moist, dark, no air circulation) then the fungus will quickly expand into irregular shaped blotches that will cover every petal.
Leaves get irregular black sections (peonies are famous for this)
Infected flower buds won’t open (on all plants not just roses) and if your flower bud won’t open – suspect botrytis.
Stems might have slightly sunken lesions spreading down the stem from infected flower buds. This fungus also produces cankers on stems similar to other fungal problems. It is not uncommon to see entire stems killed back. This is particular true in susceptible varieties or plants where the conditions are excellent for fungal problems and no controls are taken. Think extended periods of cool, humid weather and you’re looking at prime fungal development on rose flowers.
How Did My Plants Get It?
As befits this most common of fungus problems, spores are lighter than air and are carried along on the air currents. If you happen to have a flower that is badly infected and the fungus has moved to the fruiting stage, moving the flower will produce a “cloud” of fungal spores that is easily seen. Spores are common when the temperatures are between 60 and 72F and the area is moist.
How Do I Stop It?
The number one control procedure is sanitation. If you remove spent flower blossoms (called deadheading) you’ll be taking away the prime source of food for this fungus.
Clean up the garden in the fall. Botrytis overwinters on plant debris and flower stalks.
Remove all spent flowers or petals from the ground. When you see peony blossoms falling on the garden and quickly turning brown, you know botrytis is doing its thing.
Remove dead flower parts.
You have to have good ventilation in your garden.
There’s little point in crowding plants so tightly together and not expecting fungus problems.
Give plants the spacing they deserve so they can grow properly without fighting for every inch of air space.
Air circulation dries plant leaves and flowers and dry conditions will not support fungus.
As soon as you see problems,
Get out the pruning shears and cut off the diseased flower parts.
A little remedial pruning will go a long way to helping control the spread of the fungus.
Discard all infected flower parts into the garbage and do not compost them.
Do not water in the evening.
This allows water to stay on plant surfaces overnight. This high humidity is excellent for the fungus.
Water first thing in the morning so the water will evaporate from leaf surfaces.
Have healthy plants.
Plants that are fed with compost and compost teas will have better immune systems than plants fed a high nitrogen chemical fertilizer. Overfeeding produces lush, soft growth that is a signal for fungal infestations and insects to attack the plant.
Use Organic Sprays
You probably want to know what to spray the plant with to stop the problem totally.
There are fungal fighters in the environmental arsenal that will effectively knock back botrytis.
1) Lime sulphur is readily available at garden centers and this is easily applied (although it stinks) and extremely effective.
2) There are now mixes entering commercial agriculture with other fungus sprays. You spray a good fungus on your plant that attacks and eats the bad fungus. These are not yet available on a home scale but are coming.
3) Trichoderma harzianum, with a brand name Root Shield Home and Garden can be sprayed to help prevent Botrytis Blight. It is also very effective in preventing root and bulb rots. This microorganism colonizes plant tissue surfaces where it out-competes the bad fungi for space on the plants. I have not trialed this material.
4) Neem oil is used for a wide variety of plant insect and fungal problems and is highly recommended if used according to directions.
5) Make an spray of 1 tsp. baking soda and 1/4 tsp. horticultural oil mixed in 1 qt. warm water. Add a few drops of liquid soap to help the oil and water mix. Constantly agitate the sprayer while spraying to keep the oil, water and baking soda in suspension and spraying equally.
6) I note that there is research out there that shows that the baking soda and the hort oil can be used separately as well as in combination.
Do not use the same spray every time you spray.
Pick two or three sprays and alternate them. This will help prevent the botrytis from becoming resistant to your controls.
You’ll need to spray every week or after rains to keep this problem in check.
But the biggest thing is your gardening skills.
Feed the plants properly so they’re healthy and keep your garden neat and botrytis infections will slow right down.
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