Diseases of Vegetable, Ornamental and Spice Crops

FUNGAL DISEASES - I

1. LATE BLIGHT

Late blight is the most destructive disease of potato throughout the world. It was first recorded in the beginning of nineteenth century in the Andes Mountains of South America. The fungus moved from this place to Europe around the year 1842 and established in Ireland where it caused severe epidemic in 1845-46, resulting in famous Irish famine. Four million people were directly affected by this epidemic and many (approx. 1.5 Million) among them faced death with millions migrating to other parts of the world. Since then, the disease has been reported from almost all parts of the world wherever potato is grown. In India, the disease was first reported from Nilgiri Hills in between 1870 and 1880. Since then, it is appearing in plains as well as in hills regularly. In Himachal Pradesh, disease was first reported from Kumarsain of Shimla District by Butler (1903).

Symptoms:

• The symptoms of the disease appear on leaves, stem and tubers.
• Initially small water-soaked lesions develop near the tips and margins of the leaves which rapidly grow into large, brown to purplish black, necrotic lesions under favourable weather conditions (Plate 1a).
• During morning hours, whitish downy growth of the pathogen consisting of sporangiophores and sporangia can be seen on the edges of the lesions mostly on the underside of the leaves.
• Light brown to dark brown lesion appear on stem and petioles which may elongate later and girdle the affected parts (Plate 1b.)
• Since the disease is polycyclic in nature, the entire crop in the field may be killed in one or two weeks and field give blighted appearance (Plate 1c)..
• The tubers may get infected by rain washed sporangia from the diseased foliage.
• The infected tuber show irregular, small to large, slightly depressed areas of brown to purplish skin which extend deep into the internal tissue of the tubers (Plate 1d).
• The infected tuber tissue which is firm and dry in the beginning is often invaded by secondary pathogens, mainly bacteria both in the field or poorly ventilated storage places resulting in to soft rot of tubers.

Pathogen:

• The disease is caused by Phytophthora infestans (Mont.) de Bary.
• The mycelium is coenocytic, endophytic, hyaline, branched and intercellular.
• Sporangiophores arise from the internal mycelium through stomata and are slender, hyaline, sympodially branched, indeterminate, relatively thick walled, septate having side branches with swollen base (Plate 2).
• Sporangia develop on the tip of the sporangiophores and are hyaline, thin walled, lemon shaped, distinctly papillate.
• On maturity, the sporangium turns to the side, growth of the sporangiophore continues which looks to have zigzag growth with characteristics swellings at the nodes.
• P. infestans is heterothallic for sexual reproduction and involves two mating types namely A 1 and A 2.
• In India, although A 1 mating type is of common occurrence, prevalence of A 2 type has been recorded in Shimla Hills after 1984.
• Antheridia and oogonia are produced after A 1 and A 2 mating types come in close contact. Antheridia are amphygynous while oogonia are spherical.
• Oospores are thick walled and develop after fertilization.

Disease cycle and epidemiology:

• The fungus perpetuates as dormant mycelium in the infected tubers in the cold storage or in infected tubers left in the field at temperatures <30^o C (Fig. 1).
• Infected tubers transported from hills may also carry latent infections.
• Oospores even if formed rarely, also serve as source of primary inoculum.
• Tomato growing in the vicinity of potato may produce initial inoculum if infected early and act as collateral host.
• The infected tubers generally give rise to diseased haulms on which the pathogen sporulates during suitable weather conditions, thus providing primary sporangial inoculum for further spread.
• Cool moist conditions favour the disease spread while dry spells kill the fungus quickly.
• Relative humidity >90 per cent coupled with suitable temperature (12-24^o C) is most important for disease development.
• The sporangia germinate at temperature ranging from 2-30^o C.
• Temperature around 16-18^o C are optimum for mycelial growth while 9-16^o C are optimum for sporulation.
• At a temp. 12-13^o C, the sporangia germinates by producing zoospores while direct germination of sporangia takes place around 24^o C.
• Initial low temperatures thus, can help in assuming the high disease severity in the crop.

Forecasting:

• Several methods of late blight forecasting have been developed for better disease management through proper planning of spray programme.
• These are primarily based on record of temperature and rainfall or relative humidity and predict the probability of the late blight development assuming the presence of inoculum. “BLITECAST”, a forecasting model is being used in North-eastern U.S.A. for timing fungicide application.
• Such disease prediction rules have also been developed in Netherlands (Dutch Rules) and England (Beaumont Rules).
• In India, CPRI people achieved complete success in predicting outbreaks of late blight in the hills. According to them:-

i) the 7 day moving rainfall (30 mm for Shimla, 28.9 mm for Ootacamund and 38.5 mm for Shillong) and mean temperature of 23.9^o C or less continues for 7 days, the disease would appear within 3 weeks and if
ii) hourly temperature ranging between 10-20^o C associated with 80 per cent or more relative humidity continues for 18 h for 2 consecutive days, the blight would appear within a week.

Management:

• Use healthy seed and cull piles near cold stores should be destroyed.
• High ridges and proper earthing up prevents tuber infection.
• Restrict irrigation during cloudy days.
• Use resistant varieties like Kufri Giriraj.
• Give timely application of fungicides like mancozeb/ chlorothalonil/ propineb (0.25%) as prophylactic sprays (1st week of November for main crop in plains and IIIrd week of June in hills) and repeat at weekly intervals.
• Repeat sprays promptly after rain. Use metalaxyl + mancozeb or cymoxanil + mancozeb (0.25%) only when disease risk is likely to be severe and favourable weather conditions persist.
• Avoid the use of formulations containing metalaxyl alone.
• Digging of tubers should be done after 2-3 weeks of dehaulming.

2. EARLY BLIGHT
Symptoms:

• The symptoms first appear on older leaves as dark brown oval or angular spots or lesions surrounded by chlorotic zone, which may extend much beyond the lesion due to the presence of toxin “alternaric acid” produced by the pathogen (Plate 3).
• Under favourable climatic conditions, they enlarge rapidly, become irregular and may involve entire part of the leaf lamina.
• Concentric ridges appear on the necrotic tissue giving them target board appearance, lesions on leaf coalesce together and give blight appearance.
• Dark brown lesions are produced on stem and petioles, which break at the point of infection.
• On the tubers, the lesions are slightly dark, sunken and round to irregular in shape, with time infected tubers show dry rot symptoms.

Pathogen:

• The fungus responsible for this disease is Alternaria solani Sorauer.
• The mycelium is septate and pale to olivaceous brown.
• Conidiophores arise singly or in small fascicles, through stomata from the mycelium present in the dead centres of the spots.
• Conidia are usually solitary, obclavate, oblong to ellipsoid, tapering to a beak almost of the same length or even longer as that of the conidial body.
• Conidia are muriform, pale to golden or olivaceous brown having 9-11 transverse and 0 to few longitudinal septa.
• No perfect stage of the fungus has been recorded.

Disease cycle and epidemiology:

• The fungus overwinters as mycelium or spores in the infected plant debris.
• The pathogen also infects other hosts like tomato as well as solanaceous weeds which may play an important role in its survival.
• Primary infection occurs on older leaves early in the season.
• In general, hot and humid climate with frequent rains is favourable for disease development.
• A temperature range of 26-28^o C with 70-80 per cent RH is favourable for the development of the disease.
• The disease is often more severe when the plants are predisposed by injury, poor nutrition or other types of stress.

Management:

• Various cultural practices like removal and destruction of diseased haulms from the infected field after harvest, use of healthy seed tubers for planting and proper fertilization of the plants should be followed.
• With the initiation of the disease, spray the crop with mancozeb (0.25%) or chlorothalonil (0.2%) and repeat at 10-14 days interval.
• Resistance source is available with Solanum phureja and S. chacoense which can be exploited for breeding resistant cultivars.
• Cv. Kufri Sindhuri provides good resistance against this disease in plains.