by Nanette Londeree, Master Rosarian
Plants, whether cultivated or wild, generally grow well when the soil provides them with sufficient nutrients and moisture, sufficient light reaches their leaves, and the temperature stays within a “normal” range. However, like people, plants can get sick. Agents similar to those that cause disease in people also can cause diseases in plants. The broadest definition of plant disease includes anything that damages plant health. This definition can include such diverse factors as pathogens, insufficient nitrogen, air pollution, lawnmower or deer damage. A stricter definition usually includes any persistent irritation resulting in plant damage and characteristic symptoms. This definition includes such factors as pathogens, insufficient nitrogen, and air pollution. A very strict definition includes only infectious organisms (pathogens) that multiply and spread to other nearby plants. Most pathogens are microscopic and include bacteria, fungi, nematodes, viruses, protozoa, and parasitic plants. These are called biotic diseases. There are many other ailments that affect roses that are not a result of infectious organisms. These are called abiotic diseases or disorders. A working definition of an abiotic disease or disorder is damage caused by factors other than pathogens.
Here are some common abiotic disorders:
- Nutrient deficiencies occur when a plant lacks a required material for growth. The most common is nitrogen deficiency that causes chlorosis (see photo above) and drop of older leaves.
- Nutrient excesses are essentially an overdose of nutrients that may limit rose growth if the total salt level becomes too high. Lack of vigor, short shoots or burning at the leaf edges can result, especially during drought periods. Overfertilizing is the most common cause of nutrient excess, but can also result from animals urinating around the plant.
- Herbicide damage may appear as a variety of symptoms, including cupped, curled, or chlorotic leaves, small leaves, or necrosis of the entire plant. The herbicide class and the dosage to the plant determine which symptoms appear and their severity. Injury from glyphosate (Roundup) is relatively common. Damage symptoms caused by this herbicide may not appear during the season of application, especially if application is made in autumn, but symptoms may appear the following spring as a proliferation of small shoots and leaves from buds. The plant will outgrow the injury if the dosage was not too high. Glyphosate can be absorbed through green wood of canes, so care must be taken to avoid accidental application or drift.
- Sunburn appears as blackened areas, especially on the south and west sides of canes (see photo above). Sunburn is caused by excessive temperatures /direct sun hitting the cane. This is usually an indirect result of defoliation caused by drought stress or spider mite infestation. Reflected heat from masonry, vinyl siding, or rock mulch may also cause canes to sunburn. Sunburn is pretty common in Marin.
- Leaf scorch due to environmental conditions can be seen when temperature and moisture suddenly turns hot and dry. Leaves turn yellow or have brown leaf edges. The affected leaves may remain on the plant or drop early. Watering during hot, dry weather may prevent or alleviate leaf scorch.
- Drought is usually readily apparent with the wilting of the plant from top to bottom. Mild drought may not be as obvious – often the rose simply sulks, does not grow or bloom, but does not wilt.
- Desiccation is a result of water loss from the leaves and canes. In the spring and summer, desiccation can occur when plants are actively growing, temperatures are high, humidity is very low and water is limited. In the winter it is caused by winter sun and wind and roots in frozen soil that are unable to replace lost water. Newly planted bareroot roses are particularly vulnerable, as their roots are not yet developed.
- Frost cracking is caused by extremely rapid temperature changes in bark and wood of canes. If there is a dramatic drop in temperature (i.e., from 30 F to -20F in a very short period of time), uneven contraction of the wood causes a crack to form suddenly. Though not generally a problem in most of Marin, some open areas can get intense frost that can do damage to new rose canes.
Prevention is the key for most of these abiotic conditions. While you can’t control temperatures or wind, you can make sure your plants are adequately watered and fertilized (not too little or too much), keep pets away from the plants, and protect newly planted bareroot roses by surrounding the new plant with mulch until it has developed roots and can withstand drying winds. Keep in mind that any of these conditions can weaken a rose and allow invasion by many secondary fungi and insects.