ID’ed Botrytis on some roses

Other Yard

The little pink climbing roses on either side of the front gate bloomed heavy for weeks then all of a sudden most blooms turned blackish, shriveled and fell apart over the past 2 days.

For the past 3-4 days, some of the blooms on the two white hybrid tea roses in the Bay Tree bed have also turned black and shriveled. The look was a little different from the pink roses but also a lot the same.

Based on internet research and inspection of the damaged blooms the problem is likely Botrytis. Sprayed Mancozeb on both the pink and white ones in Bay Tree Bed.

Response by National Gardening Asso. to a similar issue as ours is below and provides a list of issues with roses that can cause brown petals on rose blooms.

Q: My rose bushes have petals that are rusty colored on the edges of the petals. What is causing this? They are blooming well, but the flowers are not very pretty due to the discoloration.

Answer from NGA May 19, 2008

It’s really difficult to diagnose a plant problem without actually inspecting the plant! But, here are the most common reasons for brown petals on your rose blooms:

Botrytis – a fungal disease that proliferates during extended wet weather. This disease may or may not affect buds, but I have seen it attack the entire plant. Additional symptoms to look for include a gray-brown mold. Botrytis is easily controlled using normal fungicidal treatments. And like most fungi, it more easily prevented than cured. So be on the lookout for any signs of disease and treat immediately.

Lack of good drainage – caused by improperly preparing the soil to an adequate depth for good drainage around the rose roots. This is the most serious problem in my part of the country – where soil is heavy in clay content. Rose roots need air. And since water displaces air in the soil, the lack of good drainage means the roots do not receive sufficient air. The result is brown-edged blooms, weak spindly stems, and “sad” leaves.

Improving drainage is best done before planting the rose. Double digging may be required. If drainage cannot be improved, then consider planting the roses in elevated beds or in containers.

Calcium deficiency – may reflect a calcium/boron deficiency or soil imbalance. This is common in roses where high nitrogen and high phosphorous sources (like those from water soluble, synthetic rose fertilizers) are used during periods of heat stress and in soils with high salt indices. A soil test is a must. If calcium is deficient, add dolomitic lime. Chances are, the problem is caused by the fertilizer used on the roses. Consider changing from a water soluble, high nitrate/high phosphorus fertilizer to an organic, slow release blend.

Phytotoxic reaction – common in roses sprayed with fungicides or insecticides during the summer. The chemicals (or their inert ingredients like surfactants) can cause browning of petal edges if applied during or just before the heat of the day. Spraying, if necessary, should be done very early in the morning. 

Thrips – the pest larva is the most common cause for brown-edge petals in the spring and early summer. The larva spends much of this part of its lifecycle scurrying about inside your rose buds sucking vital plant juices from succulent new growth. To determine if your rose has thrips, pull back the bloom’s petals with your fingers and look for small “slivers” scrambling about looking for cover.

Controlling thrips is a bit more difficult than discovering them. Controls include high-pressure watering wands designed for insect control to natural predators like lacewings and predatory mites to an arsenal of synthetic and organic pesticides.

Balling – a condition caused by high humidity or over watering from above the rose. Some roses are more susceptible than others. Blooms only partially open and petal edges begin to turn brown and rot. To determine if balling is the cause of brown-edged petals, feel and smell the bloom. If it feels slimy and smells like something that belongs in a compost bin, balling is the most probable cause.

Controlling this problem is not easily done unless it is caused by over-the-top watering, in which case the irrigation system should be changed to water from below the plant. The plants susceptible to balling will usually overcome the problem later in the season when humidity is lower.

Heat stress – a condition caused by planting new roses (usually grown in a greenhouse) into a new, full sun condition without hardening them off before planting. New growth and buds are desiccated by the hot sun conditions and very common if the roses are planted where they receive only west sun. This is most noticeable in darker roses.

National Gardening Association
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