Chilli (Capsicum annum L.) is one of the most important constituent of the cuisines of tropical and subtropical countries and the fourth major crop cultivated globally. Around 400 different varieties of chilies are cultivated throughout the globe. The hottest variety being “Carolina Reaper” developed by a grower Ed Currie of West Indies having the maximum pungency of about 2.2 million SHU (Scoville Heat Units; PuckerButt Pepper Company, 2013). One of the hot chilli varieties of the world “Naga Jalokia,” is the native of Tezpur in Assam, India. Numerous varieties of chilli are grown for vegetables, spices, condiments, sauces, and pickles occupying an indispensable position in Indian diet. Apart from the explicit importance of the crop in the diet, chilli is also used in other forms like medicines and beverages and also as an ornamental plant in the gardens. Nutrition wise these are enriched with high Vitamin A and C content; high iron, potassium, and magnesium content with the ability to boost the immune system and lower the cholesterol levels (Grubben and Mohamed El, 2004). India has been a leading producer, consumer and exporter of chilli especially in dried form. Various varieties of the crop are found in India and its quality varies among the states of the country.

Known for over 9500 years, chilli is the native of Southern America and was first cultivated in Peru at around 7500 BC (MacNeish, 1964). It has been the first ever domesticated crop of America. During the course of evolution, three important species of Capsicum i.e., C. annuum, C. frutescens and C. chinense evolved from a common ancestor that grew wildly in the North of the Amazon basin (NW-Brazil, Columbia) spreading to the other parts of America. Initially, people cultivated them with other crops in order to protect their crop from the damage caused by birds. By the end of eighteenth century five species of chilli i.e., C. annuum L., C. baccatum L., C. chinense Jacq., C. frutescens L., and C. pubescens R. & P. were domesticated in different parts of the America (IBPGR, 1983).

Introduction of chilli to India is being credited to the voyage of Columbus, who brought the seeds from Spain, introducing it to Europe, which subsequently spread to Africa and Asia (Heiser, 1976). However, Columbus confused the pungent fruits of Capsicum with black pepper Piper nigrum L. calling them as red pepper owing to the red colored fruits. Capsicum is not related to the Piper genus. The popularity of the crop extended swiftly across Europe is moving to India, China and then to Japan. It was incorporated into all the Asian and European cuisines almost instantaneously unlike the other spices introduced from the Western Hemisphere. Since its introduction, it has been cultivated aggressively in all the tropical and sub-tropical countries.

In its course of evolution staring from Central America to the whole of Europe, Africa and Asia, copious terminologies have been used to name the pungent fruits. The phraseology in case of Capsicum is however very confusing. Names like pepper, chili, chile, chilli, aji, paprika are used to denote the pungent fruits of Capsicum. The term “Capsicum” is reserved for taxonomic discussion. The vernacular names include pepper, chile, and aji. The common term used to denote chilli in India is “Mirchi” in Hindi. Bell pepper generally refers to the blocky non pungent fruits of Capsicum known as “Shimla Mirch” in Hindi.

Belonging to the Solanaceae family, Capsicum genus consists of approximately 22 wild species and five domesticated species (Bosland, 1994). Having chromosome number 2n = 24, Capsicum species may be herb or sub-shrub of height up to 2.5 m with extensively branched stem having hairy growth with purplish spots near the nodes. The tap root is strong with numerous lateral roots. Flowers are generally solitary, terminal, bisexual and pentamerous with campanulate to rotate corolla. Stamens are adnate at the base of the corolla tube with blue to purplish anthers. The ovary is superior having 2–4 chambers. Filiform style is found with capitate stigma. Chilli fruits are considered vegetable and are botanically berries (Figure 1).

Based on the fruit characteristics, i.e., color, pungency, shape, flavor, size, and their use, the types of chilli are classified (Bosland, 1992, 1996). It is a perennial crop and can be grown throughout the year. The major harvest season is between December and March with its supply reaching maximum during February-April (Bosland, 1996).

Chilli is used in all forms starting from fresh green fruits with ripe fruits along with its dried and powdered form. Fresh green pungent fruits are generally used in salads, stuffing, and as a flavoring agent in cooked meals. The non-pungent varieties are cooked as vegetables or processed with other food items for flavor (Welbaum, 2015). Very pungent varieties are consumed in small quantities generally considered as a condiment or spice for seasoning and for stimulating appetite. Hot peppers are also pickled in salt and vinegar used in ketchups as flavoring agents (Grubben and Mohamed El, 2004). Apart from its extensive use in preparation of processed food, chilli varieties are also used as coloring agents in salad dressings, meat products, cosmetics, and even clothing.

Capsicum possesses various medicinal and nutritional values as well. It is interesting to quote that fresh green chilli fruits contain more Vitamin C than found in citrus fruits, while red chilli fruits have more Vitamin A content than as found in carrots (Osuna-García et al., 1998; Than et al., 2008). The active ingredient of the spice, capsaicin is a complex of capsaicinoid alkaloids found in variable concentration in different chilli varieties. It is found in abundance in the placental tissues and cross walls of the fruits. However, in very pungent fruits, it is distributed in all the fleshy parts of the fruit (Grubben and Mohamed El, 2004). The amount of capsaicin has been the measure of the pungency of the chilli variety and is generally expressed in Scoville Heat Units (SHU) (Scoville, 1912).

Capsaicin possesses the antioxidant, anti-mutagenic, anti-carcinogenic, and immunosuppressive activities having the ability to inhibit bacterial growth and platelet aggregation. It is also used as anti-arthritic and anti-inflammatory agent. Regular consumption of chilli fruit is helpful against hemorrhoids, varicose veins, anorexia, and liver congestion. Pure and processed form of chilli extracts is used externally as rubefacient and analgesic in case of back pain, rheumatism, articular and muscular pains and swollen feet and even as antidote in case of poisoning. The two non-foods and non-pharmacological application of capsaicin are in the preparation of “pepper sprays” as weapons of self-defense and in preparation of “natural and organic pesticide” (Welbaum, 2015).

Being an important crop of tropical and sub-tropical countries, cultivated in large areas of Asia, Africa, South, and Central America and southern Europe, the total area under cultivation in the world is about 1.9 million hectares (Vanitha et al., 2013). About 42.2% (801,600 ha) cultivable land of the total area under chilli production at world level lies in India, producing approximately 21.4% (1.4 million t) of world's total chilli production (FAOSTAT, 2012). India has been the major producer as well as exporter of the dried chilli in the world (Figure 2). The other important producers of chilli include China, Mexico, Peru, Turkey, Thailand, Indonesia followed by countries in tropical Africa mainly Ghana and Ethiopia.

Chilli is a universal spice crop of India grown in almost all the states of the country. The quality of the chilli varies from state to state. For instance, chilli of Karnataka is known for its oil content, Gujarat quality is majorly known for its sharp color while that of Rajasthan is well known for making pickles. The chilli cultivated in Assam is famous for its pungency and that grown in Andhra Pradesh is mostly used as vegetables and liked by non-vegetarians. Major chilli producing states include Andhra Pradesh, Maharashtra, Karnataka, and Madhya Pradesh (Post harvest profile of chilli, 2009). Apart from being a large consumer and producer of chilli, India is also the largest exporter of the crop. Over 30% of the chilli produced in India is exported to countries of West Asia, East Asia, USA, Sri Lanka, and Bangladesh, most commonly in dried form (FAOSTAT, 2012).

Though being an important spice crop grown worldwide, many constraints decrease production, causing significant reduction in yield and seed production. Plant diseases have been a major reason for the crop losses worldwide. Diseases caused by fungi, bacteria, viruses or nematodes have adversely affected chilli production in almost all parts of the world. A summary of the various diseases associated with the host crop chilli is outlined in Table 1.

Other than the losses due to the pests and pathogens, crop loss in post-harvest conditions further add in delimiting the yield and production of the crop (Prusky, 2011). Specifically, in developing countries, the post-harvest losses are more serious owing to poor transportation and storage facilities (Sharma et al., 2009). Moreover, due to the recent strengthening of the food security norms in the international markets, the trade of food contaminated with fungal toxins, mycotoxins has been declared unhealthy for human consumption (WHO, 2002). Species belonging to Aspergillus genus like A. flavus (Bankole et al., 2004) and A. parasiticus (Garcia-Villanova et al., 2004) have been majorly kept responsible for production of harmful aflatoxins B and G, whose contamination in food commodities has led to serious health problems in human, like cancer and aflatoxicosis (Jeffrey and Williams, 2005). Mycotoxin contamination in dried chillies has limited its export to major export destinations like the UK and USA. Post harvest loss due to aflatoxin contamination in chilli has been reported to be about 20% to about 100% of samples obtained from Turkey (Demircioglu and Filazi, 2010) and Malaysia (Reddy et al., 2011).

Among the large number of diseases affecting chilli cultivation, anthracnose disease caused by Colletotrichum species, bacterial wilt by Psuedomonas solanacearum and viral diseases like chilli veinal mottle virus (CVMV) infection and cucumber mosaic virus (CMV) infection have been most detrimental to chilli production (Than et al., 2008).

Anthracnose disease has been reported to be a major constraint in chilli production in tropical subtropical countries causing huge losses. India had been the largest producer and exporter of chilli, but since a few years the production has declined significantly and presently, India stands at the third number in terms of chilli production (FAOSTAT, 2012; Figure 3). An estimated annual loss of about 29.5%, amounting whopping figure of US$ 491.67 million has been reported from India alone (Garg et al., 2014). In India, a calculated loss of 10–54% has been reported in yield of the crop due to the anthracnose disease (Lakshmesha et al., 2005; Ramachandran and Rathnamma, 2006). Significant losses have been reported from other parts of the world as well, like a significant amount of 20–80% loss has been accounted from Vietnam (Don et al., 2007) and about 10% from Korea (Byung, 2007). The loss is high owing to the post and pre harvest involvement of the pathogen causing a loss of 10–80% of the marketable yield of chilli fruits (Than et al., 2008).

The fruit lesion is the most economically important aspects of the disease as sometimes, even a small lesion on the fruit is enough to lower its market value thereby affecting the profitable yield of the crop (Manandhar et al., 1995). The disease is reported to affect almost all aerial parts of the plant. Chiefly, it causes fruit rot at both green and red stages primarily attacking ripe fruits, hence is also known by the name ripe fruit rot of chilli (Agrios, 2005). The disease is seed borne, soil borne, water borne and airborne and hence may lead to damage at the seedling stage or on the aerial parts of the plants. Many species of Colletotrichum have been associated with the pepper anthracnose in different countries. Table 2 gives the brief outline of the different Colletotrichum species reported to be associated with the anthracnose of chilli in different parts of the world. However, in India, primarily three important species, namely, C. capsici, C. acutatum and C. gleosporoides have been reported to be linked with the disease, with C. capsici Syd. Butler and Bisby causing major damage at the ripe fruit stage of the plant (Ranathunge et al., 2012; Saxena et al., 2014).

Environmental factors play an important role in deciding the severity and spread of any disease. The favorable host, pathogen and weather conditions lead to establishment of disease (Agrios, 2005). Thus, before proposing the management strategy of the disease, a thorough knowledge regarding the epidemiology of the disease should be studied. Anthracnose disease of chilli is generally most common among the tropical and sub-tropical countries. Hot and humid environmental conditions support the spread of the disease.

Other important environmental factors governing the severity of the disease include rainfall intensity and duration, humidity, leaf surface wetness and light. Amongst them leaf surface wetness has been directly linked with the severity of the disease owing to the better establishment of the pathogen in respect of germination, attachment and penetration into host tissues (Than et al., 2008). The relationship between the environmental factors like rainfall intensity and duration and the prevailing temperature and humidity along with the crop geometry and inoculum spread leads to possible development of disease as well (Dodd et al., 1992). Temperature also affects the development of the disease and presence of surface wetness and competitive microbiota further favors the disease development (Royle and Butler, 1986). Temperature around 27°C with relative humidity of 80% have reported to be the most optimum conditions for successful establishment of the disease in a given area (Roberts et al., 2001). The development of the disease also depends on the host cultivar, along with its resistance against the pathogen.

On attainment of favorable conditions, characteristic symptoms appear on the ripe chilli fruit, which appear as sunken circular or angular lesions (Figure 4A). Often multiple lesions coalesce to form severe fruit rot. Generally, the lesions are characterized by the presence of black colored spots in concentric rings at maturity. Initially, orange to pink conidial masses may be visible on the fruit surface. The dark spots when observed under a microscope are the acervuli structures containing setae hairs entrapping the conidia of the pathogen. Further, the pathogen forms micro sclerotia in plant debris or seed, soil, which is the mode of survival under unfavorable conditions. The pathogen infects all parts of the host plant, including stems and leaves (Figures 4B,C). Lesions on stems and leaves appear as small sunken grayish brown spots with dark margins, further on which development of acervuli in concentric rings could be easily seen.

Colletotrichum spp. has been rated as among the ten most notorious pathogens in the world, causing heavy crop losses worldwide (Dean et al., 2012; Figure 5). Specifically, Colletotrichum is an asexual genus belonging to phylum Ascomycete and Coeleomycetes class of Fungi imperfectii (Dean et al., 2012). Despite significant developments in studies related to this plant-patho system, the taxonomic position of the pathogen remains unclear. The systematics of the fungal pathogens from this genus still exist in ambiguity with the number of species ranging from 29 to over 700 depending upon the criteria selected for separation (Sutton, 1992).

Being economically important pathogen, its host range varies from fruits, vegetables, ornamental plants to important staple food crops. The species of this genus are reported to cause anthracnose disease in more than 121 plant genera from 45 different plant families (Farr et al., 2016). It also causes blights of aerial plant parts and post-harvest rots. The damage may extend to severe economic loss in tropical and sub-tropical countries, causing infection to staple foods like bananas, sorghum, cassavas, legumes, and cereals (Bailey and Jeger, 1992). Particularly, this pathogen exhibit efficient infection in post-harvest conditions owing to its ability to cause latent infection, where the symptoms appear on the fruits only after the harvest or during storage or at the market shelf. Losses up to 100% have been recorded due to Colletotrichum spp. (Prusky, 1996). Another important parameter for its successful colonization and severe disease spread may be credited to its cosmopolitan nature. Many species of Colletotrichum may be found on a single host or single species may be able to infect different hosts (Sander and Korsten, 2003). Broad, imprecise and often overlapping fungal plant relationships exist in Colletotrichum plant-patho system (Freeman and Shabi, 1996). With its ability to infect many hosts along with adapting to new environments, the pathogen poses serious threat to the different crop production system through cross infection problems (Photita et al., 2005). Figure 6 gives the distribution of the sexual teleomorph of the Colletotrichum in different parts of the world.

Impact on global economic loss posed by the pathogen has triggered extensive studies on diverse aspects of the biology of the pathogen for better understanding of its infection process and host interaction mechanisms. In light of this, host specificity of the pathogen (Freeman, 2000; Correll et al., 2007) along with the biology involved in the infection mechanisms used by the pathogen (Perfect et al., 1999; O′Connell et al., 2000) and the various fungal-host interactions (Prusky et al., 2000) studies have been carried out. Studies related to genetic diversity and the epidemiology have also been reported (Freeman, 2000; Timmer and Brown, 2000). The genus has been used as a model for studying the genetic basis of symbiotic life styles (Rodriguez, 2000) leading to the development of infection and disease forecasting systems (Uddin et al., 2002). The use of molecular markers like DNA fragments analysis [e.g., Randomly amplified polymorphic DNA (RAPD) and Arbitrarily primed (AP)-PCR] has improved the speed and accuracy in identification and characterization of Colletotrichum spp. (Lewis et al., 2004; Photita et al., 2005). Further the the nucleotide sequenceof the 5.8S gene and internal transcribed spacer (ITS) region has facilitated the construction of Colletotrichum species specific primers providing a rapid and accurate method for diagnostic purpose and phylogenetic analysis (Torres-Calzada et al., 2011). Many studies have been carried out to resolve the issue of species complex in Colletotrichum spp. in different areas of the world on various hosts like mango (Kamle et al., 2013), guava (Mohd Anuar et al., 2014), herbaceous plants (Photita et al., 2005), soursop (Álvarez et al., 2014), and also in medicinal plants as endophytes (Lima et al., 2012) using molecular marker analysis.

The anthracnose causing pathogen in chilli varieties have been reported to be Colletotrichum capsici (Sydow), Butler and Bisby (Than et al., 2008). Three pathotypes have been linked with the infection on ripe fruits, while two have been reported causing infection at mature green fruit stages (Mongkolporn et al., 2004; Montri et al., 2009). The convoluted relationships of the fungi with the host have left certain gaps in the knowledge of the infection process and the interactions of different species with chilli plant. Different species have been reported to be linked causing infection in different parts of the chilli plant, for instance, C. acutatum and C. gloeosporioides infect the fruits at all stages of development without causing much harm to leaves and stems of the plant, while C. coccodes and C. dematium mostly cause high infestation on leaves and aerial parts of the plant (Kim et al., 2004). Differential rates of infestation has also been reported by the species of the genus infecting different fruits of the plant. For instance, in Korea, Hong and Hwang (1998) reported C. gloeosporioides as the most prevalent species causing an infestation in chilli fruits at both ripe stages and unripe stage; while existence of C. capsici and C. acutatum as the prevalent species infectionsnfection on ripe and unripe chilli fruits respectively, in the North eastern region of India have been reported (Saxena et al., 2014).

Colletotrichum employs different strategies for causing infection to the host plant which initiate from the intracellular hemibiotrophic mode to the intramural necrotrophic mode of nutrition (Bailey and Jeger, 1992). Liao et al. (2012) has reported an intermediate stage showing partial endophytic life style of the pathogen before adapting to the necrotrophic mode of nutrition in the host plant. Different species of this genus exhibit different mechanism of infection depending on the host infected. For instance, Peres et al. (2005) reported the epiphytic or endophytic mode of survival of C. acutatum in an orchard infected with the bitter rot of apple. Also, intramural necrotrophy by C. capsici was reported by Pring et al. (1995) while infecting cowpea leading to subsequent necrosis caused due to dissolution of cell wall structures. The biotrophic phase of infection by C. capsici is also well studied in the infection caused to broad bean or lentil characterized by the presence of large multilobed, multi septate, vesicular primary hyphae (Latunde-Dada and Lucas, 2007).

Initial infection starts with the attachment of the conidia to the host surface preceded with its germination and production of adhesive appresoria followed with its penetration into the host epidermis. This is further accompanied by the growth and colonization of plant tissue by the fungus, resulting in the formation of specific symptom structures that is acervuli containing the spores of the fungus for further spread (Prusky et al., 2000). The pathogen sometimes remains in quiescent state in the form of appresorial structures in tissues of unripe fruits and cause infection after the fruits ripe or mature (Than et al., 2008). A dendroid structure composed of multiple, thick-walled hyphal branches with swollen or sharp ends from the penetration pore of the appresorium has been recently reported to be an intermediate structure which penetrates the host cuticle layer and infect the epidermal cells during C. acutatum infestation in chilli (Liao et al., 2012).

Though the pre-penetration mechanisms exhibited by Colletotrichum species appear somewhat related to each other, the post penetration events such as spore adhesion, melanization and cutinization hold certain disparity. Based on the previous studies, four kind of infection strategies with varied hosts have been studied in C. acutatum plant patho system (Peres et al., 2005). First is the biotrophic growth of the pathogen, where the formation of appresoria from the conidia is followed by the formation of secondary conidia which further infects and spreads the pathogen inside the host leaves (e.g., The biotrophic disease cycle in citrus leaves). The second is the subcuticular intramural necrotrophy with the development of wide and swollen hyphae in the anticlinal and periclinal walls of host epidermal cells (e.g., The necrotrophic disease cycle on strawberry). The third strategy is the hemibiotrophic mode of infection where the pathogenic hyphae interact with the infection vesicles within the host cells (e.g., The biotrophic disease cycle on blueberry fruits). The fourth type of interaction is the combination of hypertrophic and subcuticular intra and intracellular development of the pathogen generally observed during infestation of almond leaves and fruits.

As far as studies related to infection and colonization by Colletotrichum species i.e., C. gloeosporioides on susceptible chilli variety is considered, no biotrophic stage in the form of infection vesicle has been found during the infection (Kim et al., 2004). An increased number of small vacuole with the condensed cytoplasm in the epidermal cells followed with cell destruction extending to the sub epidermal cells of the plant due to the action of pathogen enzyme has been noticed during the early stages of infection in chilli plant. During the later stages, inter and intra cellular colonization of the pathogen occurs indicating the governance of necrotrophic mode of fungal growth. Figures 7A,B shows the diagrammatic representation and the microscopic representation of different stages of infection of Colletotrichum spp.

Management of chilli anthracnose has been a burning issue for the agriculturists and the farmers as till date, no effective control measures has been proposed. The fall in the chilli production and the drop in fruit quality have further intensified the need for developing a sustainable approach for controlling the spread of the disease. No single management technique has been found to efficiently control the disease. Generally, using a combination of the different strategies like chemical control, biological control, physical control and intrinsic resistance has been recommended for managing the disease (Agrios, 2005). The management strategies for controlling Colletotrichum spp. from spreading and establishing a disease can be discussed under four broad categories: Use of cultural Practices, use of chemical control, use of resistant varieties and finally the use of biological control. Table 3, gives a summarized information on the strategies used for controlling the anthracnose disease in chilli from different parts of the world.

Though the epidemic nature of the disease has been studied for ages, many areas are still unexplored in terms of host-pathogen interaction, its spread and effective management strategies. There lies an urgent need to develop an efficient integrated management strategy keeping in concern the different environmental factors and pathogenic resistance, driving the successful colonization of the pathogen in the host tissues. An insight into the pathogen's lifestyle would provide valuable information required to develop targets for developing resistant varieties of chilli against the pathogen. Also, modifications in conventionally recommended cultural practices suiting to a particular agro-climatic region will prove helpful in better management of the disease. More studies are required for acquiring in-depth information regarding various modes of infection by the pathogen and the pathogenic variability associated within a region with the post-harvest as well as pre-harvest loss in the crop production. The overall knowledge about the key aspects of a disease triangle will enable better management of the disease keeping track of the quality and quantity of the crop produced thereby contributing efficiently to the country's economy.

AS has drafted the Manuscript; RR, HS, and VG have critically reviewed the draft for important intellectual content and provided substantial contribution for the concept and design of the Manuscript.