Bacterial blight of cassava

Xanthomonas axonopodis pv. manihotis is the pathogen that causes bacterial blight of cassava. Originally discovered in Brazil in 1912, the disease has followed cultivation of cassava across the world.^[1] Among diseases which afflict cassava worldwide, bacterial blight causes the largest losses in terms of yield.

Hosts and symptoms

Xanthomonas axonopodis pv. manihotis is capable of infecting most members of the plant genus Manihot.^[1] Consisting of about 100 species, the most economically significant species is easily the widely cultivated woody shrub, Manihot esculenta, known colloquially as the cassava plant.^[2] In cassava, symptoms vary in a manner that is unique to this pathogen. Symptoms include blight, wilting, dieback, and vascular necrosis. A more diagnostic symptom visible in cassava with X. axonopodis infection are angular necrotic spotting of the leaves—often with a chlorotic ring encircling the spots. These spots begin as distinguishable moist, brown lesions normally restricted to the bottom of the plant until they enlarge and coalesce—often killing the entire leaf. A further diagnostic symptom often embodies itself as pools of gum exudate along wounds and leaf cross veins. It begins as a sappy golden liquid and hardens to form an amber colored deposit.^[1]

Disease cycle

Xanthomonas axonopodis pv. manihotis is a vascular and foliar pathogenic species of bacteria. It normally enters its host plants through stomatal openings or hydathodes. Wounds to stems have also been noted as a means of entry. Once inside its host, X. axonopodis enzymatically dissolves barriers to the plant’s vascular system and so begins a systemic infection. Because of its enzymes inability to break down highly lignified cell walls, this pathogen prefers to feed on younger tissues and often follows xylem vessels into developing buds and seeds. Seeds which have been invaded by a high number of bacteria are sometimes deformed and necrotic, but assays have shown a high percentage of infected seeds are asymptomatic carriers. In moist conditions, Xanthomonas axonopodis pv. manihotis has been shown to survive asymptomatically for up to thirty months without new host tissue, but is a poor survivor in soil. It persists from one growing season to the next in infected seeds and infected clippings planted as clones in fields. Once one cassava plant is infected, the whole crop is put at risk to infection by rainsplash, contaminated cultivation tools, and foot traffic.^[1][3][4] These are effective methods of transmission because they cause wounds to healthy cassava plants, and X. axonopodis uses these wounds as an entry point.

Favorable conditions

Hailing from Brazil, Xanthomonas axonopodis pv. manihotis excels in a humid subtropical to tropical climate. Cassava cultivation primarily takes place in these climates across the world, and X. axonopodis has followed it. It has been confirmed up and down Latin America into North America, Sub-Saharan Africa, Southeast Asia/India, and even Polynesia. As this geographical distribution would suggest, this pathogen requires a humid environment and warm habitat for pathogenicity. Nonpathogenic, epiphytic survival of X. axonopodis has been demonstrated under field conditions of high relative humidity, high rainfall, and high cloud cover/sun obstruction. These conditions match those of the early rainy seasons in the pathogen’s (and cassava’s) ecoregion.^[5] The favorable conditions described allow colonial growth and eventual swarm behavior to enter hydathodes, stomata, or wounds.

Management

Cultural Approaches: Transplantation of clones is the most common mode of propagation of this crop so the most important control of bacterial wilt of cassava is planting uninfected clones. In areas where bacterial wilt has not yet been established, it is important to raise a new crop from a meristem culture certifiably free of disease. In areas where the disease is already prevalent, great care should be taken to ensure clippings are taken from healthy plants and even then from the highly lignified portion at the base of cassava plants which appear healthy.^[4]
Seeds are known to be able harbor the pathogen, but successful sanitation measures have been described. Infected seed immersed in water at 60 °C showed no sign of bacterial survival while the seed showed no reduction in germination potential.^[3]
Intercropping and crop rotation have both been implemented in cassava cultivation and have been successful.^[6] In the instance of crop rotation following an infected cassava crop, deep soil turnover is recommended and a period of six months should be observed before cassava is planted again; X. axonopodis is a poor soil survivor and does not sporulate so this time frame should clear crop fields of inoculum. It is also important to clear the field of weeds as X. axonopodis, is known to survive much longer epiphytically on weeds than it does in soil.^[1]
Host Resistance: There is considerable variance in cassava resistance to bacterial wilt, and this is a promising means of control. The resistance which has been identified in South American strains of cassava works by preventing colonization of the xylems. The genes which control this resistance are currently being mapped and implementation efficiency should see uptick in coming years.^[7] Biological Control: Colombian clones of cassava normally susceptible to bacterial blight showed a yield increase by a factor of 2.7 when applications of Pseudomonas fluorescens and P. putida were applied four times a month during the growing season. This approach shows promise but requires further investigation,.^[1][3]

Importance

Cassava is a staple of the human diets in developing countries in the tropics. 2007 global production was 228 million tons, with 52% coming from Africa. It is estimated that cassava accounts for 37% of total calories consumed by humans in Africa.^[8] It has been further estimated that it provides the sixth most calories of any crop worldwide.^[6] These numbers would probably be even more impressive if bacterial blight were eradicated. Estimates for how much cassava crop Xanathomas axonopodis pv. manihotis destroys every year vary widely but studies have shown that one infected transplant can end in 30% loss of yield in one growing cycle, and up to 80% by the third cycle if no control measures are taken.^[1] There have been a number of historical outbreaks of bacterial blight. Zaire lost 75% of its tuber yield and almost all of its protein-rich leaf yield every year of the early 1970s, while parts of Brazil lost 50% of tuber yield in 1974.^[1]

References