Diseases caused by Fungi:
Leaf scorch, red fire disease, Stagonospora curtsii.
This fungus infects leaves, flower stalks, flower parts, or bulb scales. Affected plant parts are often bent or deformed at the point of infection. Distinctive red spots or blotches develop on diseased tissues. Flower stalks of plants with severe infections often dry up without producing flowers. Since the fungus is probably present in the bulb, infections occur as flower stalks and leaves emerge from the bulb.
Control can be achieved by minimizing moisture on the leaves and flower stalks by careful watering. It is also helpful to provide good ventilation and plenty of light since these will decrease the chance of spreading the fungus. Heavily infected bulbs should probably be discarded.
Diseases caused by Viruses:
Foliage appears mottled and can develop white or pale yellow spots which often turn red. Additional symptoms include stunting of the flowers and death of leaves.
Since these are systemic diseases, destruction or roguing of infected, symptomatic plants helps to prevent spread of the viruses to healthy ones. Once a plant is infected, recovery is unlikely although symptom severity and expression may wax and wane with changes in season.
Potato aphid, Macrosiphum euphorbiae.
The potato aphid, Macrosiphum euphorbiae, normally passes the winter as glossy oval black eggs on roses. The eggs hatch in the spring when the leaves first unfold. The aphids live on roses for two or three generations and then migrate to potato and other herbaceous plants. Although the infestation usually does not become noticeable in Connecticut until July, aphids are usually present in small numbers on the plants in June. If virus diseases are present, these aphids will spread them, but insecticides are not effective in stopping the spread. In the absence of chemical insecticides, aphids do not reach high enough numbers to do direct damage because natural enemies usually keep their numbers in check. See Aphid fact sheet.
Spanish moth caterpillar, Xanthopastis timais.
This brownish-black, hairless caterpillar has cream-colored lateral stripes that encircle its body. Unless a large number of plants are affected, handpicking caterpillars should give control of this pest.
Spotted cutworm, Xestia spp.
This is a nocturnal, flower-feeding cutworm that matures to 2" long. It is tan with lateral diagonal markings toward the end of the body. Using a flashlight to hand pick larvae at night should give sufficient control. During the day, they can be found hiding in the soil at the base of the plant.
Thrips, Heliothrips haemorrhoidalis and Taeniothrips simplex.
Both field and greenhouse plants are attacked by thrips. Flower buds and flowers are attacked by the gladiolus thrips, T. simplex. A few species of thrips are severely injurious, with the gladiolus thrips being the worst offender. The 1/16th inch, elongate, black adults and smaller, yellowish nymphs cause injury by rupturing tender epidermal leaf, flower and corm tissue and sucking the plant juices that ooze out. The flower buds may be injured so that no good blossoms result. Females lay eggs in slits made in leaf surfaces. The nymphs take from 2 to 4 weeks to mature, allowing several generations in one year. They overwinter in the corms.
Storing the corms at 40 - 45º F can break the life cycle. Another method is to make a solution of 1.25 tablespoons of disinfectant per gallon of water, and soak them three hours in this solution just prior to planting. The tiny insects hide in the sheaths of the flower stem where it is difficult to reach them with a spray. Among the compounds registered for use against this pest in Connecticut are methoxychlor, carbaryl, and spinosad, which can control this pest when applied early in the growing season prior to flower formation. Consult the label for dosage rates and safety precautions. Burning the old stalks and trash on the field, if allowed, may help in control of this insect. See Bulbs.