Answer: 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 are 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.
Solutions are simple. If purchasing a new container-grown rose from a nursery, set it out in a partial sun condition for a few days before planting it in its final location. Give the rose a chance to become accustomed to its new environment. Roses prefer full sun, but if given a choice between only afternoon sun and only morning sun, choose the morning sun location. Plant your roses in the fall or very early spring so they will have a chance to become accustomed to their environment before the heat of summer arrives. You can also consider spraying your rose with an anti-transpirant/anti-desiccant during the extreme heat of summer.
Hope this information helps you decide just what might be causing the symptoms on your rose flowers.
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