Shrimp plant (Justicia brandegeana or Beloperone guttata) is a native of Mexico that also goes by the name of false hops and Mexican shrimp bush. It prefers warm climates, though in some Northern states it is grown from cuttings as an annual outdoors or is grown indoors in pots. Shrimp plant tolerates a wide range of planting conditions and isn't susceptible to many diseases. The few diseases that attack shrimp plant can be avoided or treated with relative ease.
Fungal leaf spot affects shrimp plant when the weather is warm and wet, or when it is repeatedly watered on the foliage. There are several varieties of leaf spot, and most cause cosmetic, not fatal damage. Nematodes, though not technically a disease, are a microscopic pest that infests the roots of shrimp plant. Root rot disease occurs when the soil cannot drain, and the roots have little to no access to oxygen.
Tiny brown, tan or black spots on the leaves are the first symptom of leaf spot fungus. These spots may grow together until they form blotches, and in severe cases the shrimp plant may begin to defoliate. Nematodes will cause root damage that will result in dwarfing, yellowing and wilting. Root rot will cause wilting of the plant, the soil will likely be soggy and roots will appear slimy and brown.
Avoid planting shrimp plant too close in climates with frequent rain; spacing plants 2 feet apart provides air circulation that helps keep leaves dry. Interplant shrimp plant with African marigolds (Tagetes erecta) to help suppress nematode populations before they become a problem. If you plant shrimp plant annually, use African marigolds as a cover crop. Plant in well-drained soil and/or raised beds and avoid watering excessively to prevent root rot.
If the shrimp plant is defoliating from leaf spot, do not fertilize, but remove all fallen leaves from the planting area. Avoid overhead watering and wait for a new flush of growth. If shrimp plants are crowded, thin them out so air can circulate in the planting bed and keep the leaves drier. Treat nematodes with nemacide, or plant a new shrimp plant in a different area of the garden. Transplant shrimp plant in soggy soil to drier soil, and avoid watering for at least a week to allow the roots to dry out.
Fungicides are generally not necessary for shrimp plants, even when they have a fungal disease such as leaf spot or root rot. Good cultural practices are easier, less expensive, and less taxing on the microbe community of the soil. Remember, fungicides kill beneficial fungus as well as harmful fungus. There are many varieties of fungus that protect roots or encourage nutrient absorption in the root zone, such as mycorrhiza. Use fungicide only as a last resort.