Research advance and prospective of the citrus scab

Abstract: Citrus (Citrus spp.) , cultivated in tropical and subtropical regions, is one of the most important fruit in the world. China is the largest country for citrus cultivation and production. By 2016, China's citrus planting area was 2 556 700 hectares, with a production of 3 618 800 tons. Cultivated citrus plants often suffer economic losses from bacterial, fungal and viral diseases. Among all diseases found in citrus, citrus scab disease has been recognized as one of the most important diseases worldwide. Citrus scab disease was first reported in Florida in the 1880 s. Currently, this disease is distributed in the world citrus producing areas except the countries in the Mediterranean region. The first record of citrus scab disease in China can be traced back to the 1920 s, and now citrus scab diseases occur in all citrusproducing areas of China. Citrus of mandarin, lemon, and pomelo are usually more seriously affected.In generally, pathogenic fungi causing citrus scab disease include two species belong to the genus Elsinoë: Elsinoë fawcettii Bitancourt and Jenkins (anamorph: Sphaceloma fawcettii Jenkins) and Elsinoë australis Bitancourt and Jenkins (anamorph: Sphaceloma australis Bitancourt and Jenkins) . E. fawcettii causes citrus scab (formerly sour orange scab or common scab) on the fruits, leaves and twigs of many susceptible citrus cultivars, including lemons (C. limon (L.) Burm. F.) , grapefruit (C. paradisi Macf.) , satsuma (C. unshiu Marc.) and many tangerines (C. reticulate Blanco) and their hybrids. Severely affected leaves, twigs and fruits are often deformed into scab, and the fruit's acceptability and value for the fresh market was reduced due to the external blemishes. E. australis causes sweet orange scab, mainly affecting the fruit and rarely affecting the leaves of sweet orange [C. sinensis (L.) Osbeck] and some mandarin (C. reticulate Blanco) cultivars. Compared to E. fawcettii, erumpent scab pustules caused by E. australis tend to have a flatter cork appearance. E. fawcettii is widely distributed, while the distribution of E. australis is limited to humid citrus-growing areas in Southern America, such as Argentina, Bolivia, Brazil, Ecuador, Paraguay and Uruguay, and has recently been reported in some regions in the United States, such as Florida, Texas, Louisiana, Mississippi, Arizona, and California. Recently, a new pathotype, natsudaidai pathotype of E. australis on Citrus natsudaidai Hayata was reported in Korea. It is nonpathogenic to sweet orange. In addition, a strain from lemon in Brazil has recently been identified as a new species Elsinoë citricola Fan, Barreto & Crous, sp. nov. Similar to citrus scab, E. citricola affects the leaves, twigs and fruits with the similar symptoms to those caused by E. fawcettii, but the geographical distribution and host range are unknown. As a fungus of the genus Elsinoë, E. fawcettii produces a red or yellow water-insoluble pigments named‘elsinochrome'in the culture medium. Elsinochrome belongs to photosensitizing reagents which absorbs light energy and reacts with oxygen molecules to produce active oxygen species (ROS) , such as superoxides (O2-) , hydrogen peroxide (H2 O2) , hydroxyl radical (OH·) and singlet oxygen (1 O2) . ROS could cause damage to the cell membranes and electrolyte leakage from the cells. Elsinochrome is a non-host selective toxin which synthesized via the polyketide pathway and toxic to citrus cells and other plant cells. It has been proven that the type I fungal polyketide synthase encoding gene (EfPKS1) of E. fawcettii is involved in elsinochrome synthesis.Deletion of EfPKS1 completely abolished elsinochrome synthesis, and this function was restored by reintroducing EfPKS1 into △EfPKS1 restored this function. Under laboratory conditions, the synthesis of elsinochrome is regulated by a variety of environmental and physiological factors. Among them, light is not only an indispensable factor for toxicity, but also a factor for elsinochrome production. In addition, alkaline conditions are also beneficial for the production of elsinochrome. The genes required for the biosynthesis of secondary metabolites in filamentous fungi often reside in a cluster. The gene cluster response for elsinochrome synthesis was predicted, based on this prediction, its synthetic pathway of was inferred. In Elsinoë spp., elsinochrome synthesis and sporulation are closely linked. EfPKS1 or TSF1 deletion mutant of E. fawcettii not only loses the synthesis of elsinochrome, but also loses its ability to sporulate. Symptom-based diagnosis is the most economical, rapid, and commonly used method for diagnosis of citrus scab. In general, citrus scabs can be diagnosed based on typical symptoms. However, symptoms may change due to host varieties, stage of the infected organs, and field environmental factors. In addition, the symptoms of citrus canker and Botrytis-injury, insect damages, as well as wind damage and other mechanical damage are similar to citrus scab, which makes it difficult to distinguish.Elsinoë spp. grows very slowly in vitro, and it is difficult to identify Elsinoë spp. based on the morphological characteristics. Host range, pathogenicity and molecular method such as ITS sequence analysis, Amplified Fragment Length Polymorphism (AFLP) and random amplified polymorphic DNA (RAPD) can effectively distinguish these species and pathogenic types, but it takes time and labor, and requires a large amount of experimental space. Fortunately, specific primer pairs for E. fawcettii, E. australis and natsudaidai pathotype of E. australis based on PCR rapid molecular diagonosis were developed by Korea scientists. Although existing diagnostic techniques cannot distinguish the pathotype of E. fawcettii.Citrus scab can severely affect the growth of leaves and twigs, and causes fruitlet-dropping. Moreover, it can cause serious external blemishes on fruit and reduce their economic value. The disease must be controlled before the diseased fruit is destined for the fresh market. Integrated control strategies including planting disease-free seedlings, removing diseased shoots, strengthening the management of orchards and timely spraying protection. Citrus scab is manageable with appropriate fungicides spraying that achieve sustained profitability. Generally, scab control requires two or three fungicide applications.The first spray is usually applied at the beginning of new shoot germination in early spring, followed by a second application just at the time of the petals almost fallen, and a third application approximately 3 weeks thereafter. Effective fungicides registries for citrus scab control include copper fungicides such as Bordeaux mixture, copper hydroxide; benzimidazole fungicides such as thiophanate-methyl; ergosterol synthesis inhibitors, such as diniconazole, difenoconazole and myclobutanil; strobilurins, such as azoxystrobin; and the other fungicides, such as mancozeb, chlorothalonil, etc.. However, long-term use of the same systemic action mechanism of fungicides is likely to lead to the emergence fungicide-resistant biotype of E. fawcettii. For example, benomyl resistant strains were widely distributed in Florida as early as 1985, and it was also reported in Japan and New Zealand. However, the resistance of E. fawcettii in China to any fungicides remains unclear. Both E. fawcettii and E. australis are listed in regulated pests of EU, and E. australis is a regulated pest in the United States. A recent survey for the presence of E. australis in China was failed to confirm it existence. In order to prevent invasion of E. australis companying with the importation of citrus fruit, we recommend including E. australis in the list of pests that are prohibited from entering China, and implement quarantine to prevent disease input and to protect the safety of citrus production and export in China.