Impatiens, beloved for their vibrant blooms and shade tolerance, are susceptible to several common diseases that can compromise their health and aesthetics. From the plant's perspective, these infections disrupt vital physiological processes, leading to wilting, discoloration, and ultimately, death. Understanding these pathogens and their organic management is key to maintaining a healthy garden.
This is a devastating fungal-like disease. From our perspective, the initial symptoms are often subtle: a slight yellowing or stippling on the upper surface of the leaves. The most telling sign is the appearance of a white, downy growth on the undersides of the leaves. This growth consists of the pathogen's reproductive structures. As the infection progresses, leaves curl downward and drop prematurely, leaving bare stems and effectively halting our flowering process. The entire plant can collapse rapidly, especially in cool, moist conditions.
Unlike downy mildew, powdery mildew thrives in warmer, drier (but humid) conditions. We display this infection as white, powdery or talcum-like spots that first appear on our upper leaf surfaces. This fungal growth interferes with photosynthesis, our energy-making process. Infected leaves often become chlorotic (yellow), twist, and may drop prematurely. While not always immediately fatal, a severe infection weakens us significantly, stunting our growth and reducing our overall vigor and bloom production.
Caused by the fungus Botrytis cinerea, this disease attacks us in cool, wet weather. It typically enters through wounded or senescent (aging) tissue, such as fading flowers. We develop soft, brown, water-soaked spots on our petals, leaves, and stems. A characteristic grayish-brown, fuzzy mold soon covers these infected areas. This mold produces vast numbers of spores that spread the disease. Botrytis can quickly cause a soft rot, causing entire sections of the plant to collapse and decay.
These soil-borne fungal diseases attack the very foundation of our being: the root system. From our point of view, the first sign is often wilting, even when the soil is adequately moist. This occurs because the rotting roots can no longer take up water and nutrients. Upon inspection, our roots will appear brown, mushy, and rotten instead of firm and white. Stem rot at the soil line (damping-off) can cause young plants to collapse and die suddenly. These pathogens flourish in waterlogged, poorly drained soils.
Organic management focuses on creating an environment where we can thrive and pathogens cannot.
Cultural Practices: The most effective strategy is prevention. Ensure we are planted in well-draining soil and watered at the base to keep our foliage dry. Provide adequate spacing between us to promote superb air circulation, which reduces leaf wetness and humidity around our canopy. Immediately remove and destroy any infected plant material; do not compost it.
Organic Fungicides: For powdery mildew, weekly sprays of horticultural oil or neem oil can be effective. Baking soda sprays (1 tablespoon per gallon of water with a dash of horticultural oil) can alter the leaf surface pH, inhibiting fungal growth. For botrytis and root rots, products containing Bacillus subtilis or Streptomyces lydicus (e.g., Serenade, Actinovate) are beneficial bacteria and fungi that can help suppress the pathogenic ones.
Resistant Varieties: For impatiens downy mildew, the most crucial organic step is to choose resistant New Guinea impatiens or alternative shade-loving plants like begonias or coleus, as common garden impatiens are highly susceptible.
