This is a common and devastating disease for Lobelia seedlings and young cuttings. From the plant's perspective, it is a rapid attack at the most vulnerable point: the stem base at the soil line. The pathogen, often water molds like Pythium or fungi like Rhizoctonia, infects the tender tissues. The plant's vascular system is compromised, cutting off the flow of water and nutrients. The stem becomes water-soaked, soft, and thin, unable to support the plant's weight. The seedling collapses and dies very quickly. Treatment is difficult once symptoms appear, so prevention is key. Ensure excellent drainage, use sterile potting mix, avoid overcrowding seedlings to promote air circulation, and water carefully to keep the soil moist but not soggy.
Powdery mildew is a frequent foliar disease affecting many ornamentals, including Lobelia. For the plant, it manifests as a white to grayish, powdery fungal growth on the upper surfaces of leaves, stems, and flower buds. This coating interferes with the plant's ability to photosynthesize effectively, robbing it of energy. Severe infections can cause leaves to yellow, curl, distort, and prematurely drop, weakening the entire plant and reducing its flowering potential. This fungus thrives in humid conditions with poor air circulation but does not require free water on leaf surfaces. To treat and manage it, improve air flow by spacing plants appropriately and thinning out dense growth. Apply horticultural oils or neem oil, which work by suffocating the fungal spores. In severe cases, a registered fungicide containing potassium bicarbonate can be used.
Also known as gray mold, this fungal disease is a significant problem in cool, wet, and humid weather. From the Lobelia's perspective, it appears as a fuzzy, grayish-brown mold that attacks fading flowers, buds, leaves, and stems. The pathogen typically enters through senescent or injured tissue. It produces spores that spread easily and can quickly invade healthy parts of the plant, causing soft rot and wilting. The plant's tissues are broken down and destroyed by the advancing fungus. Management involves strict sanitation: promptly remove and dispose of any infected plant debris and spent flowers. Water the plants at the base to avoid wetting the foliage and flowers, and do so early in the day so they dry quickly. Ensure good spacing to promote drying air movement.
This is a serious condition often caused by oomycetes like Phytophthora or fungi like Fusarium. For the Lobelia plant, the first signs are often above ground: wilting, yellowing lower leaves, and a general decline in vigor despite adequate moisture. This is because the pathogen is attacking the root system and the crown (where the stem meets the roots), destroying the tissues responsible for water and nutrient uptake. The roots may appear brown, black, mushy, and rotten instead of firm and white. This disease is fostered by waterlogged, poorly draining soil. Treatment is challenging. Isolate affected plants to prevent spread. Improve soil drainage immediately by amending with organic matter. In containers, ensure pots have adequate drainage holes. Fungicidal drenches may be used as a preventative measure in high-risk situations, but they are rarely a cure for an already infected plant.
