Mango production is seriously hampered by many diseases particularly in recent days. About 140 disease-causing pathogens are known to inflict damage to mango crop at various stages of its production. Among them, diseases on inflorescences or blossoms, if unchecked, take heavy toll on yield of mango. Powdery mildew and blossom blight or anthracnose are the most notorious diseases infecting mango in India at the time of flowering and fruit set.
Powdery mildew affecting mango inflorescence is characterised by greyish white or whitish colour talc-like haze and it may cover the inflorescence partially or entirely, leading to reduced fruit set or complete failure of the crop. It affects the young developing fruits also. This disease is caused by a fungus named as Pseudoidium anacardii. In different developmental stages of panicle/ inflorescence, no infection occurs up to protected stage (elongated inflorescence still protected by bracts), even when the disease level reached the peak on other inflorescence in the field. When the axes of the inflorescence changes from green colour to red colour, the inflorescence become slowly susceptible to powdery mildew i.e., from the protected stage susceptibility increases and the full bloom stage is the most susceptible stage. Powdery mildew symptoms were diagnosed in the field when around 20% of the inflorescence attained red coloured and red open stage. Hence, it is better to start the fungicide application before or at the time when 50% of the inflorescence attain full bloom stage if there is a history of occurrence of powdery mildew in previous year. The flowers killed by powdery mildew will turn brown and can be crumbled easily by one’s hand. Less often, powdery and necrotic lesions will be seen on the young leaves mostly on the areas adjacent to midrib but the older leaves are resistant to infection.
Adoption of control measures assumes significance as this disease, if not controlled at right stage, may lead to total loss in yield. This disease can be managed by using integrated disease management strategy. Following of some cultural practices like pruning of diseased inflorescences/panicles at initial stages, and improved aeration and sun light penetration in the tree canopy may minimise pathogen inoculum. None of the known mango cultivars so far has been found immune to the disease. The most effective way to control mango powdery mildew on commercial cultivar is to apply well timed fungicide sprays. And it should be continued once or twice after the first spray once in 2 weeks depending upon the disease severity until fruit set. Spray of wettable sulphur 0.2% (2gm/litre of water) provides reasonable control but it may cause phyto-toxicity, i.e., it may burn flowers and young fruits if sprays are applied during hot sunny days. Further, due to increased disease pressure, sulphur could not offer satisfactory control of powdery mildew. Hence, spraying of Dinocap (@1gm/L) or Tridemorph (@1gm/L) can be done alternatively once in two weeks till fruit set stage. Recently, apart from theses some new fungicides like Hexaconazole (0.5ml/L), Penconazole (0.5ml/L), Flusilasole(0.4ml/L), Myclobutanil (1gm /L) are also proved to be effective against mango powdery mildew, which are available in different trade names in market. If this disease is at advanced stage, it may be too late for any control measures to have effect on fruit set and yield; hence timely application of fungicide is very important to protect the mango from harmful effect of powdery mildew. High pressure spraying equipment will effectively deliver the fungicide to the top of tree. But continuous use of specific systemic fungicide has to be avoided since it may develop resistance in powdery mildew pathogen population. In some regions, both powdery mildew and hopper occur together. In such cases, a combined treatment of suitable fungicide and insecticide may be recommended.
Another inflorescence disease is blossom blight and peduncle blight incited by fungus Colletotrichum gleosporioides. It occurs when blossoming coincides with wet weather virtually destroying the panicles. The symptom appears as small flecks or pin prick spots on the panicles and on opened flowers causing death of the flowers. The dead flowers turn inky black. The blossom is the most destructive phase of this disease, as it affects fruit set and ultimately the yield. The infected flowers fall-off, having the more persistent spikes on the peduncles. The severity of the disease may vary according to prevailing weather conditions. Sometime the panicles may escape infection but the fruits may be seriously affected by the disease. The symptoms appear especially during and after ripening of matured fruits. On susceptible cultivars, fruits may be infected before harvesting and may drop off the tree. The deceased spots usually merge and cause extensive fruit decay, cracking and oozing. Mostly green fruit infections remain latent (inactive) during harvest and largely invisible until ripening. This pathogen also affects emerging new foliage by producing oval or angular brown spots leaving shot hole symptom in the centre.
The strategy of integrating cultural practices along with chemical measure control this disease very effectively. The diseased leaves, twigs, blossoms and fruits should be collected and burnt to reduce the pathogen load in the field. Blossom infection can be controlled by spraying of Carbendaim (1gm/L) at 15 days interval. On susceptible cultivars the timing and frequency of fungicide applications are very critical and spray should begin when panicles first appear until fruits are of about 2 inches long at recommended intervals. Pre-harvest spray with Thiophanate methyl (1gm/L) or Carbendaim (1gm/L) or Hexaconazole (0.5ml/L) should be done in such way that last spray falls 15 days prior to harvest. Nonetheless, it is very important to note that mango growers need to be more vigilant during flowering and fruit set stages in order to protect their crop from these notorious blossom diseases. The growers' proactive approach, clean cultivation practices and timely need based application of appropriate fungicides will go a long way in suppressing the blossom diseases of mango, thereby leading to profitable harvest from mango orchard.
Mango Powdery mildew
Mango Blossom blight
Sangeetha and S. Mandal
Central Horticultural Experiment Station (ICAR-IIHR),