Reports on Plant Diseases

RPD No. 607 - Bacterial Diseases of Geranium

April 1990

Geraniums (Pelargonium species) are the most popular flowering pot plants grown in the United States. Plant diseases are one of the key limiting factors to the high-quality production of these plants in the home and commercially.

Three bacterial diseases are known to attack geraniums: bacterial blight or wilt, southern bacterial wilt, and bacterial leaf spot. Bacterial blight or wilt is the only one of these diseases currently known to occur in the Midwest.

BACTERIAL BLIGHT OR WILT

This disease, caused by Xanthomonas campestris pv. pelargonii, is a major problem of geraniums in Illinois during warm, humid weather. It has caused losses of 10 to 100 percent, depending on the cultivar.

The symptoms are often masked indoors by low winter temperatures. Bacterial blight attacks the leaves and stems of the common garden or florist's geranium (Pelargonium hortorum) as well as the ivy geranium (P. peltatum). There is resistance to bacterial blight in the Lady Washington and Martha Washington geraniums (P. domesticum), as well as P. acerifolium, pelargonium "Torento", P. tomentosum, and P. scarboroviae.

Figure 1. Bacterial wilt of geranium.

Symptoms

Disease symptoms usually appear in warm, sunny weather when the plants are under moisture stress. Leaves infected with bacterial blight show two types of symptoms:

1. Small, round, water-soaked spots form on the underside of the leaf. Several days later, the spots develop into angular, slightly sunken, well-defined lesions about 1/16 to 1/8 inch (up to 3 mm) in diameter that may be surrounded by a diffuse yellow halo. The spots eventually turn dark brown to black and become hard and dry. Leaves quickly wilt, die, and may fall off, or hang on the plant for a week or more (Figure 1). The bacteria in these leaf lesions may migrate into the water-conducting tissues, move down through the petiole and into the stem, eventually causing the plant to die from the stem rot phase of the disease.
2. A wilting of the leaf margin occurs in some cultivars, resulting in large angular (V-shaped), yellow or dead areas bounded by the veins. These leaves soon wither and hang on the petiole or drop off. Other factors, such as potash deficiency and Botrytis blight, may cause angular, dead patches, in geranium leaves; but the wilted condition is always associated with bacterial blight infections (Figure 2).

The stem rot phase of the disease is commonly called "black rot" by growers. Water-conducting tissues within the branches and stems of infected plants turn brown to black, 2 to 4 weeks after infection. At this stage, one or more leaves on the branch will usually wilt and develop angular dead areas. The bacteria soon spread from the water-conducting tissue to other stem tissues. The stem rapidly turns a dull gray to blackish brown. The roots are blackened but not decayed (Figure 3).

Plants gradually become defoliated, except for a few dwarfed leaves at the tips of the upright blackened branches (Figure 4). The stem and branches rapidly blacken and shrivel into a dry rot with all the tissues destroyed, except for the fibers and epidermis. Some plants may partially recover from initial infection and produce new, healthy-looking branches. Since these branches harbor the bacterium, such plants usually die within 3 to 6 months.

Cuttings infected with bacterial blight fail to root, the stems slowly rot, turn a dull blackish brown from the base upwards (Figure 5), and eventually die. The leaves wilt, often showing the typical symptoms described earlier. Cuttings serve as a source of infection in the propagation bed.

Disease Cycle

Infection occurs through plant openings or through wounds that are created when cuttings are taken from stock plants. A cutting knife that is not disinfected is one of the surest means of transmitting the bacteria. The bacterium survives in water-conducting and other tissues of infected cuttings that are taken from symptomless stock plants. Infected plants are symptomless during cool weather when the temperatures are below 70 F (21 C) or at very high temperatures (90-100 F or 32-38 C). The bacteria are very infectious and are spread easily from plant to plant through plant contact and contaminated propagation media, by splashing during hose watering or misting, cultural operations, contaminated cutting knives, and insects. The bacteria are spread or handling them. The organism has been isolated from whiteflies (Trialeurodes vaporariorum), which may transmit the bacteria to healthy geraniums. The organism can survive in moist soil from 3 to 6 months, or until infected plant residues are completely decayed. Symptoms and disease spread occur most rapidly at temperatures of 70 to 85 F (21-29 C) and in plants overfed with nitrogen fertilizer.

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Symptoms

The initial symptoms include a wilting of the lower leaves followed by chlorosis and finally necrosis of the leaves. The stem eventually turns brown and then black at the soil line, and the plant collapses when 1/2 to 1 inch of the basal stem is infected. The roots can also become necrotic. Flowers fail to open normally. Stem sections reveal vascular discoloration (Figure 4). The symptoms of southern bacterial wilt always progress upward from the soil line, whereas those of bacterial blight or wilt move in either direction — up or down the stem. It is unknown if infected plants can remain symptomless.

Disease Cycle

Pseudomonas solanacearum is a soilborne bacterium that may persist in soil for 6 years or more in the absence of a suitable host plant. The pathogen survives and spreads within susceptible plants and cuttings. Infection normally occurs through the roots. The organism is transmitted on infected cutting knives, in soils and other planting media, in crop debris, by splashing or flowing water and by insects. P. solanacearum has a very wide host range.

Symptoms develop rapidly at 75 to 104 F (23-39 C), and plants die within 2 to 3 weeks under these conditions, whereas it may take 5 weeks for similar development of bacterial blight or wilt.