Tuesday, 3 September 2019

Bacterial Diseases of Vegetable Crops

Bacterial Diseases of Vegetable Crops

Bacterial Diseases of Vegetable Crops

POTATO

Bacterial Wilt of potato

Bacterial Wilt of potato


Pathogen: Ralstonia solanacearum

Synonyms: Pseudomonas solanacearum


Common name: Brown rot, Southern wilt, sore eye or jammy eye Bacterial wilt (BW) is one of the most destructive diseases of the potato, and the wilt pathogen has a very wide host range. It is a serious problem in many developing countries in the tropical and subtropical regions of the world. It is usually found between the latitudes 45°N and 45°S. It has been recorded in all Australian states except Tasmania.


Symptoms


The aboveground plant symptoms include wilting of one to two leaves on young plants during the heat of the day. Such plants tend to recover at night. On large-leafed plants, only the tissue on one side of the midvein may wilt. This is very characteristic for plants such as Nicotiana. Affected leaves turn yellow and remain wilted after a time. The area between leaf veins dies and turns brown. Usually, the main stem of the affected plants remains upright even though all the leaves may wilt and die (Photo 8.1).

Foliar symptoms of bacterial wilt on potato.

PHOTO 8.1 Foliar symptoms of bacterial wilt on potato. (Photo courtesy of A.M. Varela, International Centre of Insect Physiology and Ecology

Internal symptoms include light tan to yellow-brown discoloration of the vascular tissue. Long sections of infected stems reveal dark brown to black streaking in the vascular tissue as the disease progresses. As invasion proceeds, the pith and cortex of the stem become dark brown. Typical symptoms include wilting, yellowing, and some stunting of the plants, which finally die right back. Wilting is first seen as a drooping of the tip of some of the lower leaves similar to that caused by a temporary shortage of water. At first, only one branch in a hill may show wilting.

Affected leaves later become permanently wilted and roll upward and inward from the margins. The wilting then extends to leaves further up the stem and is followed by a yellowing of the leaves. This yellowing, wilting, and in-rolling of the leaves make diseased plants very obvious, especially when surrounded by healthy plants. The leaves finally turn brown and fall off, beginning at the base of the stem and continuing upward.

Symptoms in the tuber are very specific: brownish-gray areas are seen on the outside, especially near the point of attachment of the stolon. Cut tubers may show pockets of white-to-brown pus or browning of the vascular tissues (Photo 8.2) 

Cut tuber symptoms of bacterial wilt on the potato.

PHOTO 8.2 Cut tuber symptoms of bacterial wilt on the potato. (Photo courtesy of A.M. Varela, International Centre of Insect Physiology and Ecology

which, if left standing, may exude dirty white globules of bacteria. As the disease progresses, bubbly globules of bacteria may exude through the eyes; the soil will often adhere to the exuded bacteria, hence the name “sore eyes” or “jammy eyes.” Signs of the Pathogen Slimy, sticky ooze forms tan-white to brownish beads where the vascular tissue is cut. When an infected stem is cut across and the cut ends held together for a few seconds, a thin thread of ooze can be seen as the cut ends are slowly separated. If one of the cut ends is suspended in a clear glass container of clean water, bacterial ooze will form a thread in the water.


Host

Bacteria Ralstonia solanacearum attack almost 200 plant species in 33 different plant families. These include economically important hosts such as tobacco, potato, tomato, eggplant, pepper, banana, peanut, and beans. Thorn apple and nightshade are two common weed hosts that are attacked by the disease pathogen. This constitutes one of the largest known host ranges for any plant pathogenic bacterium. Although the Solanaceae (potato family) contains the greatest number of susceptible species, many other dicots and a few monocot plants are also susceptible.

The common name for the diseases this organism causes varies with the host that is attacked. In tobacco, it is called BW or Granville wilt (for Granville County, North Carolina where it was observed as early as 1880) and moko disease in banana. It is sometimes called southern wilt or southern BW (in the northern hemisphere). This bacterium is noted for diseases caused outdoors inland areas bounded by 45°N and 45°S latitudes where rainfall averages above 100 cm/year, the average growing season exceeds 6 months, the average winter temperatures are not below 10°C, the average summer temperatures are not below 21°C, and the average yearly temperature does not exceed 23°C. It can be moved from such areas into the greenhouse industry in and on plants propagated in those regions and then sold to growers throughout the world. Although the primary location of survival in the environment is in crop and weed hosts, it can also survive in soil. It can be readily spread through the movement of contaminated soil and infected vegetatively propagated plants, in contaminated irrigation water, on the surfaces of tools (cutting knives) and equipment used to work with the plants, and on soiled clothing.

Pathogen

The bacteria were first named Bacillus solanacearum. After several revisions, it was called, for many years, Pseudomonas solanacearum. The latest revision has settled on the name Ralstonia solanacearum. It is described as a nonspore-forming, Gram-negative, nitrate-reducing, ammonia forming, aerobic, rod-shaped (0.5–1.5 µm) bacteria with one polar flagellum. Populations within this genus and species can be further divided into races and biovars based on different host ranges, biochemical properties, susceptibility to bacteriophages, and serological reactions. Bacteria Ralstonia solanacearum (earlier known as Pseudomonas solanacearum), based on the type of host plants it attacks, is divided into three races and based on its biochemical properties, it is divided into four biovars. The most widespread strain in Australia is race 3/biovar 2. This strain is known to occur in New South Wales, Queensland, and it primarily attacks potato. Two other strains, which attack other hosts besides potato, are confined to the Northern Territory and Queensland.

Race 1 exist in the United States, where it attacks many floricultural and vegetable bedding plant crops including geraniums (all Pelargonium), Catharanthus, Impatiens, Ageratum, Chrysanthemum, Gerbera, Tagetes, Zinnia, Salvia, Capsicum, Lycopersicon, Nicotiana, Petunia, Solanum melongena (eggplant), Tropaeolum (nasturtium), and Verbena. Race 3 is tropical in distribution and does not occur naturally in North America. Race 3 biovar 2 (R3B2), inadvertently introduced to the United States in vegetatively propagated geraniums grown in Central America and Africa, is considered to be a major threat to agriculture in the United States because it causes brown rot in potato (Kim et al., 2003). Some of the other known hosts of R3B2 include Pelargonium, tomato, peppers, eggplant, bean, and beet. Weed hosts include black nightshade, climbing nightshade, horsenettle, Jimson weed, purslane, mustard, lambsquarters, and bitter gourd. The bacteria can infect through roots and through any fresh wounds. The bacterium can be difficult to work within the laboratory because it quickly loses pathogenicity and viability in artificial culture.

Disease Cycle

Potato wilt bacterium is a soil-borne organism primarily inhabiting the roots. It enters the root system at points of injury caused by farm implements, nematodes, and by other means. It is spread by irrigation water, floodwaters, and contaminated soil. BW of potato is generally favored by temperatures between 25°C and 37°C. It usually does not cause problems in areas where the mean soil temperature is below 15°C. Under conditions of optimum temperature, infection is favored by the wetness of the soil. However, once the infection has occurred, the severity of symptoms is increased with hot and dry conditions, which facilitate wilting.

The wilt bacterium is able to survive for periods up to 2–3 years in bare fallow soils and for longer periods in soils cropped to nonsolanaceous crops. Infected seed is an important method of dissemination, both locally and over considerable distances. Heavily infected tubers are not a problem since they generally rot away, but the contamination of the land in which they were grown poses a problem. However, slightly infected tubers, which show no visible symptoms, pose a serious threat of spreading the disease to new areas. Self-sown potatoes are extremely difficult to eradicate and, if a paddock is infected, the disease may remain in it for 5 or 6 years after the initial outbreak.

Bacteria can also be spread to clean tubers from an infected seed cutter. There is also a real danger of infection if second-hand bags are used or if half tonne bins have held infected potatoes. Growers should be aware of these risks and take precautionary measures.

Economic Importance

BW is responsible for causing considerable losses to the potato industry where the disease exists. In the south east of Victoria, it has caused considerable losses in the past to the potatoes planted mainly in the swampy areas. However, the threat of the disease is potentially significant to the seed potato production industry. Some states and countries that import potato regard BW in the same light as black wart, ring rot, and potato cyst nematode and ban imports from areas known to be infected. The disease can cause total loss of a crop and restrict the use of land for potato production for several years.

Disease Management

BW is difficult to control (or eradicate) because of the soil-borne nature of its causal organism. Therefore, the following options should be considered in managing the disease. Growing and propagating from pathogen-free plant material is the main way to avoid problems with Ralstonia, regardless of the race and biovar involved. Propagators must use pathogen-free potting soil or other media, establish stock plants that are tested and known to be free of the bacteria, train workers handling the stock plants in methods and procedures that prevent the pathogen from contaminating the potting soil or coming in contact with the stock plants, and then maintaining this system throughout the propagation phase of crop production.
There are no chemicals or biological agents that adequately control these bacteria. Infected plants must be discarded as soon as possible.
• Adopt rotations with pastures, cereals, and nonsolanaceous crops for periods exceeding 5 years.
• Use certified seeds from reliable sources. Exclusion of the disease may be exercised by quarantine or other legislative measures. For example, Tasmania, which so far has not recorded BW, is very careful to import only healthy seed. New Zealand and South Africa ban the importation of seed from areas known to have the disease.
• Planting in areas where BW has not occurred previously.
• Control self-sown potatoes.
• Control weed hosts such as nightshade, thorn apple, Narrawa burr around dames, along channels and in the paddocks after cropping potatoes.
• Avoid deep plowing—the organisms survive in the deep, cool layers of soil.
• Irrigation water should never be allowed to run freely over or below the soil surface. It should never be allowed to return to the dam or stream from which it is pumped nor to any other irrigation source.
• Regularly inspect crops for disease symptoms and remove and destroy diseased plants, tubers, and immediate neighbors.
• Use stock to clean up chats, discarded tubers, and crop debris, but do not allow the stock back onto clean paddocks.
• Do not return potato waste, for example, oversized, misshapen, and diseased tubers to paddocks, so as to minimise the spread.
• Machinery taken onto a diseased paddock should be left on the paddock while it is being worked.
• Machinery removed from the paddock should then be washed clean with a disinfectant
solution in a dedicated area where equipment is washed.
• Use high-pressure wash to clean machinery, sheds, and so on to remove soil adhering to any surfaces.
• Clothing and boots of people working in the paddock should be exchanged for clean items when leaving the paddock, or else boots should be washed in a suitable disinfectant.
• After harvest, all diseased and discarded tubers should be collected and buried at least 1 m underground.
• On no account should any of the produce from a diseased crop be kept as a seed?
• Load and unload vehicles only in designated areas with the sealed or hard ground or bare paddocks away from potato paddocks.
• Choose transport routes that minimize travel through potato paddocks and regions.
• If second-hand bags or half tonne bins have been used to hold potatoes, these should be thoroughly washed and disinfected before being used again. Bags should be disinfected or discarded.
• Ask visitors, contractors, and workers to wear overalls, gumboots, and overshoes on the property.

Action in the Event of Suspect Cases

Seed and ware growers, potato merchants, and importers are requested by the Department of Agriculture to examine potato tubers regularly for signs and symptoms of the disease. If the plant's tubers are suspected to be infected, please contact your local plant health inspector or contact the Department of Agriculture, Fisheries and Food.

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