Characterization of Kinnow Mandarin fruit blemishes and tree-fruit- environment (TFE) profile in relation to various blemishes causal agents

Kinnow mandarin (Citrus nobilis Lour×Citrus deliciosa Tenora) fruit skin blemishes have become the key concern for growers and exporters. The objective of this study was to characterize various fruit skin blemishes, and to develop profile of the dynamics of blemishes in relation to the fruit growth, environmental conditions and various blemishes causal agents. Blemish characterization was done on the basis of visual symptoms, while its development was studied at various phenological stages of the fruit growth and development in relation to insect, pests and disease attack. Blemishes caused by red scales, thrips, mites, citrus canker and anthracnose were characterized among biotic causal agents while wind blemishes were characterized among abiotic causal agents. April and May were found critical months in which, maximum insect pest activity was observed with highest blemish development (36% and 31%) respectively. Overall, wind related blemishes (41.66%) were found maximum in the orchard followed by mites (19.89%). Monthly increment in fruit diameter was maximum (26.98 mm) during month of June. A total of 3724.08 degree days were estimated for Kinnow mandarin from fruit set to maturity (optimum fruit quality). The study would help the researchers, growers and exporters to identify and develop understanding for better management of Kinnow fruit skin blemishes.


Introduction
Pakistan currently ranks ninth among the mandarin producing countries of the world [1]. Approximately about 80% of Pakistan's citrus production comprises of mandarins; predominantly "Kinnow" and according to an estimate approximately 95% of the world "Kinnow" is being produced in Pakistan [2]. "Kinnow" mandarin has got tremendous export potential for fresh consumption. Presently about 250,000 tons of "Kinnow" fruit are being exported mainly to Russia, Middle East and South East Asia [3]. "Kinnow" mandarin cultivation has got significant boost in the country due to its high yielding character. It is appreciated by the consumers both in domestic and international markets due to easy peel, natural deep orange color, good blend of sugars and acids, high juice contents and excellent flavour. Therefore, Kinnow of Pakistan is very popular in different markets worldwide. However, in spite of having good characters, our Kinnow mandarin has lower average price, mainly due to the presence of blemishes on its skin. Pakistan is fetching a prices of $222/ton for Kinnow mandarin in the international markets as compared to other mandarin exporting countries e.g. Morocco; $707/ton, Spain; $1200/ton and China; $504/ton [4]. Now a day, skin blemishes have become a major concern for the growers and exporters as fruit quality is downgraded, with result of high farm gate rejection and lower international market prices. External presentation of fruit is an important feature to catch consumer choice in the market [5] and attractive, blemish free fruit are desired by the consumer [6]. Morphologically, the blemishes are collapsed hypodermis tissues related to shrunken or destroyed oil glands [7]. Various abiotic (wind, splitting, nutritional imbalance, sun burn) and biotic (insects, diseases and certain microorganisms) factors have been reported to cause blemishes in citrus [8,9]. Previously, information was lacking about blemishes identification and their development in relation to different phenological stages in Kinnow mandarin, since no study was reported in Pakistan on this aspect. So the objective of this study was to characterize various types of skin blemishes of Kinnow mandarin and to develop profile of the dynamics of fruit blemishes in relation to the fruit growth and development and various blemishes causal agents.
Also for better estimation of harvesting time, heat units (degree days; DD) were calculated for Kinnow mandarin.

Materials and Methods
In first stage, nature and characterization of fruit blemishes was assessed based on symptoms [10][11][12]. In order to have a comprehensive account of the type of blemishes, fruit samples were collected from orchards as well as processing units from district Sargodha. Fruit were photographed and symptoms were noted and differentiated. In second stage, fruit blemishes development was studied in relation to tree phenological stages and environmental conditions. This study was conducted in a commercial orchard in district Sargodha (32°05'N 72°40'E), which is a major "Kinnow" growing area. All the plant growth and developmental stages were observed during complete growing season. For this purpose, ten healthy trees were selected randomly in the orchard, considering each tree as a replicate. Data regarding different aspects like fruit growth and development, incidence of blemishes on the fruit, time of insect pest attack and disease incidence were recorded from individual tree. Moreover, data regarding the temperature and humidity of the selected orchard was also recorded to relate the environmental effects on fruit growth and development and ultimately on the development of the blemishes. The experiment followed Randomized Complete Block Design (RCBD). Temperature and humidity measurement A digital data logger (Tiny tag; TGP-4500, Gemini data loggers, (UK) Ltd.) was installed over a Kinnow tree in the experimented field for the whole growing season of (2009-10) to record the data regarding temperature and relative humidity (RH).

Heat unit calculation
Heat units were calculated by the method described by Hardy and Khurshid [13] using the formula: Daily Heat units= [(maximum temperature + minimum temperature) ÷ 2] -Base temperature A temperature of 13 ºC was used as base temperate.

Fruit diameter
Fruit diameter was recorded on monthly basis from the selected trees with the help of digital vernier caliper. Fruit diameter (mm) was recorded from 100 randomly selected fruit; 10 fruit each from 10 tree and average was computed.

Monitoring of incidence of insect pests and diseases
Observation of the presence of the insect pests on the selected branches of the selected tress was monitored on the Kinnow trees to relate the development of blemishes with incidence of insect pest and diseases. Blemish development For this purpose, 100 randomly selected fruit; 10 fruit each of 10 trees, were observed for the incidence of blemishes at monthly interval (April to July). Percentage of fruit blemishes in the orchard was calculated by the formula: Blemish percentage= (Number of blemished fruit ÷ total number of observed fruit) × 100

Nature and extent of blemishes in the orchard
At harvest fruit were assessed for nature and extent of blemishes. The fruit were categorized according to the nature (Biotic and abiotic factors) and extent of blemishes (<1cm 2 , 1-5%, 6-10%, 11-25%, 26-50% and >50% blemish area covered on fruit skin [9]. The percentages of fruit were calculated in each category.

Characterization of Kinnow fruit blemishes
Various types of blemishes were identified in Kinnow mandarin and their characterization with respect to various causal agents is described below.

Red scales
The blemishes caused by Red scales were identified by the presence of reddish brown small, rounded, hard scales on the skin of the Kinnow fruit (Annexure 1; Plate 1). Major damage occurs to fruit but twigs and leaves also got affected. The scales damage was observed from June to October.

Thrips
The blemishes caused by thrips were characterized by silvery ring on the stem end or calyx end of the Kinnow fruit. Sometime this ring also extended longitudinally from calyx end (Annexure 1; Plate 2). The damage mainly occurred to fruit. Thrips also destroyed the newly emerging flower buds, resulting low fruit set. Mites Mites were another major biotic factor which caused damage to the skin of the fruit in Kinnow orchards. These blemishes can be characterized by the brownish to reddish spots or irregular patches on skin of the fruit due to mites feeding. Mites feeding may be severe, infecting whole fruit or damage might be low infecting a portion of the skin (Annexure 1; Plate 3). Major damage occurred to the fruit, hence deteriorating the quality of the fruit.

Citrus canker
Citrus canker is a bacterial disease, caused by a bacterium Xanthomonas axonopodis pv. Citri. [14] and causes major damage to the fruit. The symptoms were characterized by formation of corky, blackish to brownish lesions on the skin of the fruit. These spots downgrade the quality of the fruit and make it non exportable (Annexure 1; Plate 4). Anthracnose Anthracnose is caused by fungus called Colletotrichum gloeosporioides [15]. Anthracnose was characterized by sunken, black lesions or superficial, reddish brown discoloration. (Annexure 1; Plate 5). The major damage occurred to the fruit causing quality and quantitative loss.

Wind blemishes
Wind blemishes were the most common fruit blemishes found in the Kinnow orchards. The blemishes are characterized by irregular and uneven scratches found on the skin of the Kinnow fruit (Annexure 1; Plate 6).

Stylar end deformity
The problem is characterized by breakdown of peel from stylar end of the fruit in rounded pattern (Annexure 1; Plate 7).

Branch rubbing of fruit
Branch rubbing is also common in Kinnow orchards especially if orchard is not pruned regularly. Fruits near to branches may be exposed to rubbing causing severe scratches or may rupture to the skin of the fruit (Annexure 1; Plate 8).
Poor tree/canopy management The fruit become trapped in between the branches of the tree. The fruit becomes misshapen/deformed and loses the quality (Annexure 1; Plate 9).

Tree-fruit-environment (TFE) Profile
A Tree-Fruit-Environment (TFE) profile was made to relate the Kinnow fruit skin blemishes development with environmental conditions (Fig. 1 (Fig. 2). Fruit growth and development Gradual increase in fruit diameter was observed after the fruit set to harvesting. Significantly higher monthly increment in fruit diameter was recorded during the month of the June (26.98) mm followed by July (10.18) (Fig. 3). Other months showed more or less similar and steady increase in fruit size.  (Fig. 4).

Nature and extent of blemishes in the orchard
Regarding nature of blemishes, wind blemished fruit (41.82%) were found significantly higher in the orchard followed by mites which produced 19.89% of blemished fruit (Fig. 5). Blemishes caused by wind, mites and physical were found in orchard under study, while no fruit were found blemished with thrips and citrus canker. Regarding extent of blemishes, significantly higher percentage of fruit (35%) were found in 1-5% blemish area category, followed by <1cm 2 blemish category with 30% fruit (Fig. 6).

Discussion
Fruit were characterized according to visual symptoms and it was found that symptoms of blemishes due to red scales, thrips, mites, citrus canker, anthracnose and wind, on Kinnow mandarin were similar to, as described previously by Cardwell