Dear Helen: This summer, our potted dahlias are afflicted with mildew, while the dahlias in the open garden are free of the fungus. Why would this be?
Dear M.E.: Powdery mildew appears as a whitish, powdery growth on leaf surfaces. It most commonly appears on susceptible plants in late summer as nights become cooler and more humid while days are still warm. Spores germinate rapidly in humid conditions. Enclosed or shaded areas without free circulation of fresh air facilitate the initiation and spread of this disease.
I’ve also observed powdery mildew appearing soon after the plants, expecially plants in containers, have suffered the stress of dry soil.
In your case, the in-garden dahlias may well be free of the disease simply because the potted plants’ soil is harder to keep adequately and consistently moist in hot, dry weather.
Although humid conditions favour the rapid spread of the disease, powdery mildew spores differ from other fungus diseases in that they are able to infect plants even in very dry weather with low humidity levels.
One simple control method is to hose the plants off about mid-day, a few times each week. This washes spores that are forming off the leaves and minimizes infections. Powdery mildew spores cannot germinate in the presence of water. The timing is aimed at making sure the plants don’t go damp into the night.
Clean up all top growth meticulously at the end of the season. At that time, consider unpotting the tuberous roots, cleaning them well, and storing them with a light dusting of sulphur and covered with vermiculite or wood shavings in a cold but frost-free place for the winter. A fresh start in new planting mix in the spring should help. Use large enough containers to ease the work of keeping the soil mix consistently moist. As the plants develop, consider starting up the water washes early, as a prevention measure.
Dear Helen: My potted tomatoes, growing in full sun with a planting mix that contains dolomite lime, have started producing ripe fruits, but a significant number of them have blossom-end rot. I thought the lime would provide sufficient calcium to prevent this condition.
Dear E.V.: It is well known that adequate supplies of calcium (via lime) in the soil will help to prevent blossom-end rot, which appears as dark, sunken areas at the bottom (blossom) end of the fruit. But that's just half of the story.
Even if there is enough calcium in the soil, it can become unavailable to the tomato plants in conditions of stress that inhibit the movement of calcium within the plants. The principal stress usually involved here is inadequate or uneven supplies of moisture in the soil. This is most common when plants are grown in containers, which need such frequent watering in hot, dry weather.
Another stress sometimes occurs when the process of transportation of calcium within a plant cannot keep up with rapid growth spurts, which can result from an excess of nitrogen in the soil.
Where tomatoes must be grown in pots, use the largest possible containers for easier maintenance of even soil-moisture levels. Water with careful regularity, especially in hot weather.
Dear Helen: A honeysuckle that has grown well for several years started out fine again this year. Then, suddenly, its leaves turned greyish before going almost white and dropping off. This left the plant looking peculiar as it bloomed last month on bare stems.
Dear J.H.: You don’t mention any wilting of the foliage, stem die-back, or leaf curling — symptoms that would occur in the case of most diseases and common insect infestations. Nor do you mention observing any insects.
Because of the extremely hot, dry weather in July, I suspect that spider mites may be the problem. They and their eggs are tiny enough to be not easily seen. They suck leaf juices, causing a gradual discolouration, usually accompanied by a motley and generally grubby appearance. In severe infestations, the leaves die and drop off the plant.
Spider mites thrive and proliferate mightily in hot, dry conditions. The best line of defence is to increase humidity by keeping the soil moist, mulching with a nourishing compost and spray-misting the plant frequently.
Encourage natural enemies of spider mites by avoiding the use of insecticides and growing food sources in the garden for beneficial (pest-devouring) insects like syrphid (hover) flies and lady beetles. Some of the best flowers for this purpose are cilantro (coriander), dill, yarrow, sweet alyssum and lavender.