Preventing fungal disease on rose bushes

Experiencing a summer of high temperatures, fires, smoke, ash and Santa Ana winds, residents look forward to having some relief.

The weather has moderated slightly, and along with the change comes cooler nights with more moisture collecting on leaves. This moisture with the daily accumulation of ash and small dust particles provides a great environment for mildew, rust and black spot on roses.

Black spot is the most common and important disease of roses found everywhere roses are grown. The disease does not kill the plant outright but, over time, the loss of leaves can weaken the plant making it more susceptible to other stresses and to winter damage.

It first develops on upper leaf surfaces, later adjacent areas turn yellow and leaves drop prematurely, usually beginning at the bottom of the plant progressing upward.

A potential "lookalike" disease is spot anthracnose or shot-hole disease; it is not a major problem unless temperatures are very hot – too hot for black spot. Spots caused by black spot are fuzzy around the edges, then turn yellow and brown. Spots caused by anthracnose are smooth edged and the centers turn gray and drop out. Treatment is the same – a fungicide – but it must be labeled for black spot or anthracnose, whichever disease you are treating.

Fall brings warm days, cool nights, moisture and air dust particles. These conditions will ensure large colorful blossoms and possibility of powdery mildew. One of the earliest signs will be slight purple splotches on the underside of leaves and white powdery spots on top and white powder on the peduncle, or neck, of the rose blossom. A good fungicide should be applied at the first sight of a white appearing substance on the leaves.

Fall is a good time to check the pH of the soil. It should be between 6.0 and 6.5. Any reading below or above these levels will inhibit roses ability to use the nutrients you are giving them. Treating the pH problem now will give ample time for adjustments before spring pruning. The most likely result will be a low pH due to the acidifying effect of fertilizers.

Roses benefit from a good rinsing to remove accumulated dust: be sure to keep moisture off the blossoms to prevent yet another fungal disease, Botrytis, which will appear as rot of blossoms and will usually prevent them from opening. Another sign is red blotches on blooms.

In October, I included an article about chilli thrips and pictures for identifying the problem they cause. Continue to investigate for these pests and treat them if found. They attack new growth, buds and blooms. Left untreated plants are stressed greatly, often shriveling the end buds or preventing bud formation. The life cycle of chilli thrips includes falling to ground and becoming a grub and reappearing when temperatures warm up next year.

If you completed the light midseason pruning in September or October, you pruned out dead, crossing canes and thinned the middle of the plant. It will improve air circulation through the bush and reduce possible fungal diseases.

This midseason pruning and fertilizing encourages a new blooming cycle. Feel free to cut some of the early blooms now and take them inside for bouquets.

Unless you plan to exhibit, I do not recommend fertilizing after mid-October, but you may make a final application of fertilizer for the year before mid-November. If you do this, use a fertilizer lower in nitrogen (N) and higher in phosphate (P) and potassium (K); that is, if your fertilizer has an N-P-K number on it, the first digit will be lower than the other two. If it lacks an N-P-K, read the ingredients and/or ask your professional nursery person for guidance.

To explain: nitrogen encourages foliage growth – something we want to discourage as the plants go into their winter dormancy; phosphate helps build root structure and resistance to stressful conditions (e.g., cold at this time of year); Potassium is a helper of phosphate and aids in bloom quality. If you use an organic fertilizer it will be readily available when the soil warms, adding to the nutrients needed for that spring growth spurt. A liquid fertilizer as the last application will be readily available.

Remember to check your garden daily for any changes. Be sure to keep them hydrated for best results. The weather forecast for the next week is around the mid-80s. The cooler temperatures can be misleading. Roses still need to be watered, perhaps not as often.

Some people think Southern California lacks distinct seasons, but we do have seasons. They are only discerned by those with a more sophisticated palette. So get out of the house and enjoy the subtle delights of the air, sun, and the rich aroma of our magically misty fall.

This time of year many nurseries and garden stores are liquidating their remaining inventory of potted roses – and you're in luck because November is an ideal time to purchase and plant! Make your list of new roses and go shopping; if you plan to replace an old, tired plant, prepare the area now for easier planting later. And assess your stock of fertilizers and be sure to order next month from the San Diego Rose Society.

For more ideas, visit Temecula Valley Rose Society's Rose Haven garden at 30592 Jedediah Smith Road, Temecula, as well as TemeculaValleyRoseSociety.org/index.html.

Unless you plan to exhibit, I do not recommend fertilizing after mid-October, but you may make a final application of fertilizer for the year before mid-November. If you do this, use a fertilizer lower in Nitrogen (N) and higher in Phosphate (P) and Potassium (K); that is, if your fertilizer has an N-P-K number on it, the first digit will be lower than the other two. If it lacks an N-P-K, read the ingredients and/or ask your professional nursery person for guidance.

To explain: Nitrogen encourages foliage growth-something we want to discourage as the plants go into their winter dormancy; Phosphate helps build root structure and resistance to stressful conditions, such as cold at this time of year; Potassium is a helper of Phosphate and aids in bloom quality.

If you use an organic fertilizer it will be readily available when the soil warms, adding to the nutrients needed for that spring growth spurt. A liquid fertilizer as the last application will be readily available

Remember to check your garden daily for any changes. Be sure to keep them hydrated for best results. The weather forecast for the next week is around the mid-80s.The cooler temperatures can be misleading. Roses still need to be watered, perhaps not as often.

Some people think Southern California lacks distinct seasons, but we do have seasons. They are only discerned by those with a more sophisticated palette. So get out of the house and enjoy the subtle delights of the air, sun and the rich aroma of our magically misty fall.

When you have a moment to spare, feel the need to get away or when the day cools down, take your favorite beverage and a picnic basket and visit Rose Haven Heritage Garden, 30592 Jedediah Smith Road, in Temecula. The cross street is Cabrillo Avenue. The early morning and late afternoon sunlight across the pass is magical this time of year – it even makes the freeway seem a little bit romantic.

Oh, one last thing to do when it gets just a bit too nippy out there: Start perusing rose catalogs, both printed and online versions, for that next "gotta have" rose variety. The September/October America's Rose Garden issue has a good section on some new or recent roses and Bob Martin's annual compilation of the newest roses.

Also, this time of year many nurseries and garden stores are liquidating their remaining inventory of potted roses – and you're in luck because November is an ideal time to purchase and plant. Make your list of new roses and go shopping, if you plan to replace an old tired plant, prepare the area now for easier planting later. And assess your stock of fertilizers and be sure to order next month from the San Diego Rose Society.

For more ideas, visit TVRS at http://TemeculaValleyRoseSociety.org/index.html.

Until next month, Happy roses to you!