Tag: mildew

  • Mildew-resistant lilacs

    Mildew-resistant lilacs

    Bloomerang lilac (Photo from Proven Winners)

    This summer has been considerably more comfortable, unlike the past several years with six to eight weeks of hot, dry weather that stressed me as well as my plants. There has been enough rain, allowing me to relax a bit from the daily watering ritual. Plants have grown well, but so have the spores of moisture-dependent seasonal mildews. Mildew spores are active in late summer when night air is cool and humid, and day air is somewhat drier and warmer. If we get a run of dry days and moist, humid nights, the scene will be set for powdery mildew. I won’t be surprised to see the familiar white flush on my older lilacs later this month.

    Powdery mildew isn’t really something to worry about if your plants are generally healthy. It’s one of the many fungal pathogens essential to the process of decomposing plant fibres into humus, and every active compost pile has a thriving mildew colony. Mildew is disfiguring to lilac foliage, but won’t cause any immediate damage to plants. The greatest damage is the slowing of photosynthesis and inhibited growth. But if lilacs are growing in adverse conditions — such as in low light and poor air circulation, and in soil with inadequate drainage and — repeated bouts with seasonal mildew can contribute to the lilac’s eventual demise. Seriously afflicted plants exhibit a powdery white flush on twisted foliage, as well as premature leaf drop. Trying to fight mildew infection requires more work and attention than you might want to devote to the problem (see below). A simple solution is to plant mildew-resistant cultivars, and not encounter the problem.

    My newer lilac cultivars, such as pale blue ‘Miss Kim’ (Syringa pubescens ssp. petula ‘Miss Kim’) and the reblooming purple Bloomerang (S. x ‘Penda’) seem unaffected by mildew symptoms appearing on my older lilacs in the garden. Other resistant cultivars are dwarf lilac ‘Palibin’ (S. meyeri ‘Palibin’) and the group of late-blooming Preston lilacs (S. x prestoniae), including pinkish-purple ‘Donald Wyman’, bright pink ‘Miss Canada’, rosy purple ‘James McFarlane’ and lavender ‘Minuet’. The dwarf Fairy Tale Series of lilacs, including reddish-pink Tinkerbelle (S. ‘Bailbelle’), pale pink Fairy Dust (S. ‘Baildust’) and lilac mauve Sugar Plum Fairy (S. ‘Bailsugar’) are also mildew resistant.

    Older cultivars developed from S. x hyacinthiflora and S. vulgaris are deeply scented, but unfortunately prone to mildew infections. However, there are some resistant S. vulgaris cultivars, including double purple ‘Charles Joly’, double white ‘Madame Lemoine’, blue ‘President Lincoln’ and creamy yellow ‘Primrose’.

    Crabapple trees, privet hedges and roses can also be affected, as well as summer phlox, tuberous begonias, snapdragons and zinnias.

    Fungal diseases respond best to preventative treatment, beginning before symptoms are present. Starting in early summer, weekly sulphur or baking soda sprays (1 tbsp./15 mL of baking soda and ½ tsp./3 mL of liquid dish soap mixed with 1 gallon/4.5 L of water) can help to prevent infection. Spraying twice monthly with summer grade horticultural oil (follow package instructions for mixture) is also effective. Treating plants with advanced mildew symptoms is difficult, but if you must, try a copper fungicide sold as bordeaux mixture.

    My advice on dealing with powdery mildew is simple: Remove plants that are chronically affected and replace them with mildew-resistant cultivars. And that will be one less thing to worry about.

  • Dealing with mildew on phlox

    Dealing with mildew on phlox

    'Katherine' phlox (Photo by Garden Making)
    ‘Katherine’ phlox (Photo by Garden Making)

    I’ve had an early onset of mildew on summer phlox (Phlox paniculata) this year. These scented, long-blooming perennials are one of the backbones of summer gardens, providing a broad range of colours from midsummer through to first fall frost. Mildew pathogens infect the stems, flowers and foliage, growing quickly to make phlox clumps seriously unsightly. When phlox is severely infected with mildew, it’s also likely to show up on asters, bee balm, lilacs and roses.

    Mildew first appears as a powdery coating on foliage. The spores are wind-borne and reproduce rapidly in humid conditions, particularly when combined with warm night temperatures. If weather conditions remain warm and humid for long periods, the white powder can gather into solid splotches, eventually covering leaves in thick, white fuzzy patches. Foliage will begin to drop off at the bottom, working upward. Although mildew won’t kill the plant outright, it seriously weakens it, and repeated seasons of attack will eventually cause the plant to die.

    Thinning up to a third of the stems increases air circulation and reduces humidity, making conditions less encouraging for mildew to grow. Planting phlox in full sun and avoiding overhead irrigation also keeps foliage dry in normal seasons (as long as there isn’t frequent rain and elevated humidity). Spraying plants before infection is noticed with an organic anti-fungal mixture (such as one tablespoon/15 mL of baking soda and one teaspoon/ 5 mL of insecticidal soap or horticultural oil mixed into a gallon/1 L of water) can also help to prevent mildew from getting started.

    But the most effective way to avoid mildew disease is to plant resistant phlox cultivars. Mildew infections can be quite dramatic when they have a strong hold on phlox plants, and that nasty experience has motivated breeders to work on building resistance in the plants. The good news is that there are many lovely phlox cultivars with inbred ability to resist mildew spores. The more cautious news is that geographical climate conditions have quite an effect on determining the strength of inbred resistance. Phlox paniculata ‘Delta Snow’ and ‘Robert Poore’ have been tested in university trials and show strong mildew resistance in Arkansas and North Carolina, but would they be resistant in southern Ontario climate conditions? Those southern and western locations are quite different from our northern region, and there’s less certainty of mildew resistance. And what about ‘Orange Perfection’, ‘David’ and ‘Prime Minister’, three cultivars that show disease resistance in Vermont and New York? Those northeastern states are closer to our own region, and seem a safer bet for Ontario gardens.

    I’ve had problems with mildew on phlox in my garden and came to the conclusion that they had to be removed. I replaced them with ‘Katherine’ (lavender with white eye), ‘Orange Perfection’ (coral orange) and ‘David’ (pure white), three mildew-resistant cultivars that have never been infected in my garden. Locating mildew-resistant cultivars for our region may be a bit frustrating, but there is some help. The Chicago Botanic Garden is located in a Great Lakes region, with an Illinois climate somewhat similar to my own in Ontario. Extensive phlox trials were conducted in the botanic garden over a nine-year period, resulting in a list of resistant cultivars to look for at nurseries and in plant catalogues (see https://www.chicagobotanic.org/downloads/planteval_notes/no35_phloxpaniculata.pdf ). The plants might have other growing problems (such as poor cold hardiness or weak stems), but scored high for resistance to mildew. The cultivars from that study showing excellent disease resistance are: ‘Becky Towe’, ‘Bartwelve’, ‘Candy Floss’, ‘Flower Power’, ‘Frosted Elegance’, ‘Goldmine’, ‘Lichtspel’, ‘Natural Feelings’, ‘Peppermint Twist’, ‘Pleasant Feelings’, ‘Rainbow’, ‘Rubymine’, ‘Sherbet Cocktail’, ‘Shortwood’, ‘Swirly Burly’ and ‘Wendy House’. These are some cultivar names to watch for, and perhaps they will put an end to mildew on our lovely phlox plants.

  • Monitoring downy mildew on impatiens

    Monitoring downy mildew on impatiens

    Fortunately, New Guinea impatiens is resistant to downy mildew.  (Photo from Veseys.com)
    Fortunately, New Guinea impatiens is resistant to downy mildew.  (Photo from Veseys.com)

    For years  every garden centre and corner grocery store has been flaunting flats of bright impatiens seedlings. It would be fair to say impatiens is overplanted, but their vivid colours and generous floral displays are hard to resist. Unfortunately, impatiens is being threatened by a devastating mildew disease that destroys every leaf, flower and bud, regardless of whether it’s grown in a container or in the ground.

    I’ve seen impatiens afflicted with downy mildew over the years, but never the full destruction of plants as recently reported. Infections in the United States are widespread, and the disease pathogen (Plasmopara obducens) has also been positively identified in Ontario, Quebec and British Columbia. (An Internet search for “impatiens and downy mildew” will turn up lots of information.) This strain of downy mildew is specific to both single and double garden impatiens (Impatiens walleriana), or busy lizzies as you might call them, but doesn’t infect other kinds of impatiens. New Guinea or sun impatiens (I. hawkeri) is resistant to it.

    Downy mildew is encouraged by excessive watering, high air humidity and relatively low temperatures, between 15 and 23°C. Midsummer drought conditions with low humidity and high temperatures may help to stave off mildew infections, while moist spring and late summer weather will encourage the disease. Once disease spores are present, it’s almost impossible to prevent infection from spreading. Downy mildew on impatiens is spread two ways: short-lived dispersal spores produced in the white powdery coating on the undersides of leaves are dispersed by wind and water splashes; and survival spores (called oospores) that are produced inside stems and petioles, and deposited in the soil. Spores in the soil can overwinter, and will infect new plants the following season.

    Initial symptoms of downy mildew on impatiens usually occur in moist conditions with cool nights, beginning with the youngest foliage, causing leaves to appear yellowed, slightly chlorotic or stippled, and often with a thin, white downy coating on the leaf undersides. The white powder can also be found on the undersides of leaves still green. Affected leaves fall off, leaving bare stems.

    Fungus infections are difficult to control, and the only effective treatment is preventative. A preventative spray program every two weeks from the day of planting until the end of the growing season is required to provide protection before fungus spores become established on foliar tissue. A mixture of one tablespoon (15 mL) light horticultural oil (available at garden centres) combined with one tablespoon of baking soda per gallon (4L) of water, sprayed on all sides of the leaves and stems every two weeks, is effective in preventing infection.

    Impatiens plants that are produced in greenhouses are likely disease free when they arrive at the market. However, once planted in the open garden, weather conditions may trigger the deadly downy mildew spores. What should we do about this? Will it be necessary to ban impatiens from the garden? Well, I’m still going to plant them, and certainly hope for the best. But unlike other years when I’ve given little regard to planting conditions and still had gorgeous flower displays, I’ll now give more consideration to how I plant my impatiens.

    I can’t control air temperature, but perhaps I can do something about water and humidity. This year I won’t set impatiens plants so close together as I have in the past. Leaving more space between individual plants will help to lower humidity and increase air circulation. I won’t water impatiens from overhead, but instead will flood the ground and avoid wetting the foliage. Any sick plants will be immediately removed and disposed of in garbage; and during fall cleanup, all impatiens plant parts – stems, flowers, and root balls – will be lifted, but not composted (mildew spores can live in compost).

    This is something like treating impatiens with all the fuss of hybrid tea roses. But I’m willing to keep growing impatiens, just so long as they’re healthy and happy in my garden.

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