Temperature extremes injure many plants. Browned flowers and marginal leaf burn can be obvious freeze-related damage that typically appears in late March and early April. Here are some tips to distinguish freeze injury from disease.
First, look at the pattern of damage on plants. Freeze injury is uniform. All leaves (or flowers) of a certain age are usually affected. One side of a plant that is more exposed may be affected while the rest appears normal. Brown blotches or tears appear in the most exposed or tender portion of leaves which are often the tips and margins. Uniform leaf speckling may be present.
Next, look for signs the actual organisms that cause diseases are present. Many of these are microscopic and may need laboratory confirmation. Fungal fruiting structures embedded in cankers, spore masses and bacterial ooze may be the easiest signs to find without a hand lens or microscope.
Knowledge of a disease and its life cycle can also help distinguish freeze from disease. Fireblight is the most misdiagnosed disease, often confused with freeze damage or fungal blights. Fireblight only occurs on plants in the rose family that form pome-type fruits. If the affected plant is not one of these hosts, then the damage could be freeze injury.
For help with plant problem diagnosis, contact one of the Colorado State University plant diagnostic clinics.