Isolating a plant ailment is confusing at times, especially distinguishing between plant nutritional issues and plant viruses. Even when the cause of the problem is traceable to a mineral deficiency, it’s important to identify which mineral is deficient in order to select the proper remedy.
Roses need regular fertilizer applications throughout the peak growing season: all that flower production takes up a lot of energy! (Roses also need plenty of water, because nutrients are only available to plants when they’re in solution.) In addition to the usual nitrogen-phosphorus-potassium combinations, roses also seem to appreciate extra magnesium and iron compared with other flowering shrubs. If your rose fertilizer doesn’t include magnesium, just do an internet search for “roses+epsom salts” and you’ll find formula suggestions offered by many rosarians. Iron, on the other hand, is a micronutrient that is best applied in chelated form. If your plant is suffering from iron deficiency, you may have a soil pH that’s more alkaline than the recommended growing range of 6.0-6.5 or you may have an excess of free phosphorus in the soil. In either case, iron is less available to the rose. A foliar spray with chelated iron or chelated micronutrients, including a drop or two of dishwashing liquid to help the minerals adhere to the leaves, should work well. Make sure the air temperature is between 65ºF and 85ºF when applying, and follow the instructions on the package for number of tablespoons per gallon of water.
The photo at right shows a few of the discolored leaves from my ‘Morden Snowbeauty’ rose. In spite of the patterned bleaching, I eliminated rose virus as the problem for several reasons:
- The appearance of rose mosaic virus is quite distinctive, showing up as either ringspots or saw-toothed demarcations (see photo below left).
- Rose virus is extremely rare on an own-root rose, especially when purchased from a reputable supplier.
- The leaf discoloration on the ‘Morden Snowbeauty’ is only affecting the newest leaves, not the entire plant.
The third reason is also a key in diagnosing nutrient deficiencies. Macronutrients–those nutrients found in higher amounts in plant tissue, such as nitrogen, phosphorus, potassium, and magnesium–are mobile within the plant. Nitrogen, for example is first taken into older leaves; but when newer leaves need nitrogen, the nitrogen moves from the older leaves to the newer leaves. As long as there is sufficient nitrogen in the soil to meet the needs of both older and younger leaves, the plant will continue to look green and healthy. However, if the older leaves begin to turn yellow, more nitrogen needs to be added to the soil.
By contrast, micronutrients, such as iron, zinc, copper, and manganese, are not mobile within the plant, so deficiencies show up on newer, younger leaves. Micronutrients are less available to plants when soil pH is higher than 7.0. Also, iron and zinc can become unavailable in soils with too much free phosphorus, because they bind to the phosphorus.
Ordinarily, newly planted perennials and shrubs shouldn’t be fertilized the first year in order to avoid fertilizer root burn. Regular watering is usually sufficient to get new plants off to a good start. However, after speaking with Carol at Northland Rosarium, I was assured that a weak fertilization and an application of chelated iron would probably be safe, especially since the plant is definitely demonstrating nutrient deficiency symptoms.
A quick trip to the hardware store ensued, where I located a very satisfactory organic (non-burning) 4-3-2 rose fertilizer by Espoma (that even includes magnesium) and a bottle of Fertilome Chelated Micronutrients to correct what is likely an iron deficiency. Even mature roses shouldn’t be fertilized in our zones much past the end of July, so I’ll be applying these corrective measures this week and then looking forward to a healthier rose as the summer progresses.