Introduction
From 2014 to 2016, there will be 795 million chronically hungry people in the world. Furthermore, the global population is expected to surge well over nine billion by the end of 2050. As a result, the agricultural sector will have to produce 60 percent more food globally within the same period. At the same time, roughly one-third of food produced – 1.3 billion tonnes per year – is lost globally throughout the supply chain (Food and Agriculture Organisation of the United Nations, 2015).
For decades, bacterial soft rot has caused more crop loss worldwide than any other bacterial disease. The disease can be found on crops in the field, in transit and in storage or during marketing. In total, this amounts to approximately 100 million dollars lost every single year (Bhat, Masood, Bhat, Bhat, Razvi, Mir, Akhtar, Wani and Habib, 2010). Bacterial soft rot is caused by several types of bacteria, most commonly species of Gram-negative bacteria, Pectobacterium and Pseudomonas. The soft rot causing bacteria chiefly attack succulent, tender tissues of storage organs such as fleshy tubers, fruits, roots, bulbs, corms, and rhizomes, as well as bud, stem, petiole and leaf stalk tissues (Babadoost, 1990). The host range is large, and includes genera from nearly all plant families.
Fungal soft rot is also a serious disease that is affecting postharvest crops. Fungi causing soft rot diseases include Penicillium expansum and Sclerotinia fructigena (on apples) and Rhizopus (on sweet potatoes) (Moore-Landecker, 1996). In the case of Rhizopus spp. Rhizopus, the sporangiospores of the fungi secrete pectinolytic and cellulolytic enzymes which break down pectin in the middle lamella and cellulose of the cell walls respectively, eventually causing soft rot. (Agrios, 1997). Another common disease in cabbages is the gray mould rot caused by the fungus Botrytis cinerea (Acedo, 2010).
The bacteria Pectobacterium carotovorum subsp. carotovorum (formerly known as Erwinia carotovora subsp. carotovora) and Pseudomonas fluorescens cause the most common and destructive soft rots (Agrios, 1997). Umunna and Anselem (2014) reported that bacterial soft rot is one of the most common diseases in potatoes. Other agents of diseases in potatoes are virus, fungi and pests, but bacterial soft rot alone accounts for about 30% to 50% of this loss. Furthermore, according to Acedo (2010), bacterial soft rot is the most important postharvest disease in cabbages and another study conducted by Johnson (1999) has proved bacterial soft rot to be one of the most severe postharvest diseases of potatoes worldwide.
Pectobacterium carotovorum subsp. carotovorum (formerly Erwinia carotovora subsp. carotovora) is a Gram-negative bacterium responsible for bacterial soft rot in various commercially important plants including potatoes, carrots and cabbages. It is a member of the family Enterobacteriaceae, along with other plant pathogens such as Erwinia amylovora and human pathogens such as Escherichia coli, Salmonella spp. and Yersinia spp. (Toth, Bell, Holeva and Birch, 2002). It enters plant tissues primarily through wounds, often created by insect feeding or bruising at harvest. Insects and water are effective in spreading the bacterium. Once in the plant tissue, the bacterium produces increasing amounts of enzymes that break down the pectic substances of the middle lamella, eventually causing breakdown and maceration of the tissues (Gupta and Thind, 2006). This results in damaged crops and lowered crop yields.
In order to be sustainable, agriculture must meet the needs of the present and future generations. Sustainable development has been defined from Our Common Future, also known as the Brundtland Report, as development that meets current needs, without compromising the ability of future generations to meet their needs (WCED, 1987). Sustainability often consists of three dimensions: environmental, social and economic sustainability. By reducing the spread of bacterial soft rot, sustainability can be ensured environmentally, socially, and economically.