New Guinea impatiens x hybrida (I. hawkeri) are hybrids of the original impatiens. They are both considered annuals. Annual are plants that complete their entire life cycle in one season. According to the United States Department of Agriculture (USDA), impatiens are hardy in zones 2 to 10. The New Guinea varieties are bigger and hardier than other impatiens. The New Guinea impatiens are also susceptible to different diseases, including those that are caused by different fungi.
The New Guinea hybrid is a sturdier plant and has much bigger flowers than the regular impatiens. The flowers come in a variety of colors and can grow up to 3 inches in size. Their leaves are green, bronze or variegated depending on the variety. New Guineas prefer partial shade, but are more tolerant of the sun than the original impatiens. The plant can grow to 4 feet high. They prefer rich, well-drained soil.
Most New Guinea impatiens are grown in hanging baskets or as potted plants for transplanting into the garden, or as container plants for window boxes or deck and patio plants. For garden use, placement is important because color combinations can be overpowering.
Several types of fungi diseases affect New Guineas. Botrytis blight, also referred to as gray mold, is a fungal disease that attacks New Guineas. The fungi can survive winter on dead plant debris, and in the spring, the spores start to form again. It is spread by wind, water or infected plants.
Root rot is a disease originally thought to be caused by fungi Pythium spp. or Phytophthora spp. These fungi have now been reclassified as oomycetes or plant pathogens. Root rot occurs if the soil or base of the plant remains soaked for long periods of time.
Powdery mildews are host specific with different fungi affecting different plants.
Botrytis blight will inhibit buds and flowers from developing normally and cause them to turn brown. Flowers will have sporadic brown spots with older ones rotting faster. Brown spots will also appear on the leaves and stems. All infected plant parts will become covered in a gray mold after cool, wet weather.
Root rot caused by Phytophthora spp will affect the plant by appearing as if it is drought-stressed. The plant will begin to wilt and the leaves will turn yellow. The symptoms on the roots vary depending on the type of Phytophthora spp, the host and the soil conditions. Typically the roots turn brown to black and rot.
Powdery mildew symptoms are consistent throughout all types of fungi. They can affect all types of plants and all plant parts. Powdery mildew causes spots or patches of white to gray, powder-like growth. The disease is visible on the leaves, stems, buds and flowers. The affected leaves become misshapen and drop off early. Affected buds may not open. Tiny fruit-like spots appear white, then turn yellowish brown, and then black. These are the part of the fungus that survives the winter. Powder mildew is more likely in warm, dry conditions.
Fungi Management and Prevention
Manage conditions before Botrytis blight develops. This involves allowing enough space between plants to let air flow between them. Cut off and throw away all infected plant parts as soon as they are discovered. Over-watering and over-fertilizing help lead to Botrytis blight, so it is recommended not to water from the top down onto the leaves and to avoid fertilizers with too much nitrogen. If infection is severe, fungicides may be used every 10 days.
Root rot caused by Phytophthora spp can be managed by good watering habits. Do not over-water or allow plants to be in standing water. Check frequently to make sure the soil is not too wet. If the disease has started, controlling involves avoiding the movement of infected soil, water or plant parts from one area to another. If necessary, the chemical fungicide fosetyl-al (Aliette) can be used on ornamental plants. Consult local extension offices or garden centers before using chemicals.
Powdery mildew can be prevented, or if plants are infected it can be treated chemically. Prevention involves avoiding late-season fertilizing, watering from the top down, removing infected plant parts, and pruning to prevent overcrowding. If the disease becomes severe, fungicides such as neem oil (Shield-All), triforine (Ortho Funginex) or potassium bicarbonate (Kaligreen) can be applied. Consult local extension offices or garden centers before using chemicals.
- Missouri Botanical Garden Kemper Fact Sheet: Ten Best Annuals for Shade
- Auburn University: New Guinea Impatiens
- NC State University: Commerical Production of New Guinea Impatiens
- Colorado State University Extension: Powedery Mildews
- University of California Agriculture and Natural Resources: Phytophthora Root and Crown Rot in the Garden