Botrytis: Black spots on leaves or grey fuzz on flowers/fruit

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 petals is usually 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 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?

Sanitation

The number one control procedure is sanitation. If you remove spent flower blossoms (called deadheading) and dead leaves 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 spen leaves and flowers from the ground.

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.

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.

Having said that – good old gardening advice – there is some research running around the Net suggesting when you water doesn’t make any difference.

Have healthy plants.

Plants that are fed with compost 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.

Caution:

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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