Encyclopedia
Root rot diseases remain a major global threat to the productivity of agricultural crops. They are usually caused by more than one type of pathogen and are thus often referred to as a root rot complex.
- fungi
- oomycetes
- root rot
1. Introduction
Root rot diseases remain a major global threat to the productivity of agricultural crops. They are usually caused by more than one type of pathogen and are thus often referred to as a root rot complex. Fungal and oomycete species are the predominant participants in the complex, while bacteria and viruses are also known to cause root rot. Incorporating genetic resistance in cultivated crops is considered the most efficient and sustainable solution to counter root rot, however, resistance is often quantitative in nature. Several genetics studies in various crops have identified the quantitative trait loci associated with resistance. With access to whole-genome sequences, the identity of the genes within the reported loci is becoming available. Several of the identified genes have been implicated in pathogen responses. However, it is becoming apparent that at the molecular level, each pathogen engages a unique set of proteins to either infest the host successfully or be defeated or contained in attempting so.
2. Overview of Root Rots
Root rots have a significant impact on global crop production [1]. Depending on the causal agent, host susceptibility, and the environmental conditions, crop losses can range from slightly above the economic threshold to losing complete fields [2][3][4][2,3,4]. Interestingly, legumes seem to be the most common host for these pathogens [3][5][6][3,5,6]. However, monocots and dicots, cereals and legumes, fruit trees, and tubers also fall prey to root rots.
Fungi and oomycetes most commonly cause root rot disease. However, bacteria and even viruses can be causal agents [4][7][8][9][10][11][12][4,7,8,9,10,11,12]. Due to more than one pathogen’s involvement, the disease is commonly referred to as a root rot complex. Some classic examples include the black root rot of strawberry attributed to Pythium (oomycete), Fusarium (fungus), and Rhizoctonia (fungus) pathogens [13][14][15][13,14,15], and the pea root rot complex caused by A. euteiches (oomycete), F. oxysporum, F. solani, F. avenaceum, Mycosphaerella pinodes (fungus), Pythium spp., R. solani, and Phytophthora spp. (oomycete) [16][17][18][19][16,17,18,19].
Unless the root rot complex affects seed germination, the root-specific symptoms go unnoticed or are not visible. If symptoms appear aboveground, the plants usually fail to recover. Some of the symptoms associated with root rots include browning and softening of root tips, root lesions that vary in size and color (reddish, brown, and black), yellowing and wilting of leaves, stunted plant growth, reduced yield, and loss of crop [1][3][4][20][21][22][1,3,4,20,21,22]. Selected root rot pathogens can also cause post-harvest rots in beets, potato, and sweet potato. The proliferation of root rot pathogens is favored by moderate to high soil moisture, poor drainage conditions, soil compaction, the optimal temperature for pathogen growth, mono-cropping, and other factors that contribute to plant stress [1][23][24][25][1,23,24,25]. The unpredictable climatic conditions portend an increase in mean temperatures and other natural calamities such as droughts, floods, and storms. These conditions are expected to inflict constant stress on crops, which is expected to favor the increased activity of root rot pathogens [26][27][28][26,27,28].
Cultural, physical, biological, and chemical control methods have been used as management strategies to control root rot disease. However, to date, these strategies have only been partially successful. Most of the root rot pathogens are distributed globally, and some species can survive up to 10 years in the soil [29]. Several root rot pathogens are host-specific, however, some have a wide range of hosts. Therefore, crop rotation may not be fully effective as a control method [3][29][3,29]. Chemical control is often inefficient due to these pathogens’ soilborne nature and is not the most sustainable option as it also impacts beneficial microbes. Furthermore, there is a high likelihood of cross-contamination between contiguous plots and when using shared field equipment [30][31][30,31].
There is a critical need to understand the genetic basis of root rots and incorporate the information in breeding strategies to develop root rot-resistant crops. The current understanding of plant molecular defense responses is derived primarily from studies using foliar pathosystems [32]. Specific and unique genetic and molecular aspects of the host-pathogen interactions in the roots have been unraveled in the past few decades.
