Following two diseases are discussed that cause yield loss in tobacco production.
Alternaria Leaf Spot
Frog Eye Leaf Spot
Alternaria leaf spot (Brown spot)
Brown to black spots appears with yellow halos on both sides of leaves. Later, irregular or circular necrotic areas, sometimes brittle paper-like texture.
Initially, lesions occur on young leaves as small brown to black spots that develop prominent yellow halos, often near the margin.
Lesions expand into irregular or circular necrotic areas covering large areas of the leaf. Necrosis and chlorosis may spread along the veins. Lesions are flat and visible on both sides on the leaf.
Older lesions have a brittle paper-like texture in their middle. If the corky tissue falls off, craters or pockmarks become visible.
The pathogen may affect the crop at any stage of its growth and is caused by the fungus Alternaria alternata. Even though all parts are affected, the disease infests chiefly the roots and base of the stem.
Seedlings in the nursery show black discoloration of the stem near the soil level and blackening of roots, leading the wet rot in humid condition and seedling blight in dry weather with withering and drying of tips.
The pathogen also spreads to the leaves and causes blighting and drying of the bottom leaves.
In the transplanted crop, the disease appears as minute black spot on the stem, spreads along the stem to produce irregular black patches and often girdling occurs.
The upward movement leads to development of necrotic patches on the stems. The infected tissues shrink, leaving a depression and in advanced condition the stem shrivels and plant wilts.
When the affected stem is split open, the pith region is found to be dried up in disc-like plates showing black discoloration.
On the leaves large brown concentrically zonate patches appear during humid weather, leading to blackening and rotting of the leaves.
Use healthy plant seeds from certified sources. Fertilize your crop properly to increase the plants natural resistance but avoid excessive nitrogen fertilization. Provide a good drainage system in the fields as water stress or over watering may favor disease.
Avoid overhead irrigation. Facilitate aeration by increasing plant spacing according to varietal characteristics of canopy elongation. Check your plants or fields for signs of the disease.
Crop Rotation with other leguminous crops can help reduce the levels of inoculum in a field; stalks and roots left after harvest should be removed and destroyed; control nematodes in the soil; ensure that plants have been supplied adequate potassium to promote vigorous growth.
Fungicides based on iprodione, chlorothalonil and azoxystrobin provide good control of Alternaria leaf spot. Products based on propiconazole and thiophanate methyl have also been proved highly effective.
It is important to follow the specified concentrations and to use fungicides with different mode of actions to prevent the development of resistance.
Organic fungicides show good results against Alternaria leaf spot. However, products based on copper oxychloride have been proved highly effective in controlling the disease.
Frogeye Leaf Spot of Tobacco
Small, water-soaked spots appear on leaves. Spots grow into necrotic roundish lesions with grayish centers and dark-brown margin. The infection can occur at any growth stage but it is more common on young leaves.
Over time, they grow into large (1-5 mm) roundish spots with grey centers and may be dark purple margins. In case of a severe infestation, leaves die and fall off. Elongated spots with crushed centers start to appear on stems.
The disease may also spread by airborne spores, or splashed by water onto the lower leaves; dissemination may also occur when seeds are contaminated by debris containing spores.
Survival occurs in debris, particularly on the leaves and stalks left after harvest.
Rains and high temperatures (>26oC) are ideal for fungus to proliferate and push forward spores develop a actively disease like a Frog Eye Leaf Spot.
Ensure before planting a resilient tolerant or resistant variety from a reliable certified source. Use pathogen-free viable seeds. Monitor the field regularly to observe if there is any sign of the disease.
Remove infected leaves and seedlings from the field. Provide good drainage and aeration within the plants. Try to plant earlier in the season to avoid the peak of the disease.
Conduct crop rotation of three years at least with non-host crops, such as maize and or any cereal crops.
Plow deep and bury plant residues under the soil. Remove and burn infected plants residues at a distant place away from the field.
Habitat and reproduction
Frogeye leaf spot is caused by the fungus Cercospora nicotianae.
It survives between plants or on crop residues in the field or on seeds. If planted, infected seeds may give rise to infected seedlings. Young tobacco leaves are more susceptible than the older ones.
The pathogen infects all stages of crop (even after the leaves are harvested). Initially the lower leaves exhibit brown, round lesions which resembles frog-eye shape (generally of 2 -15 mm in diameter) with grayish center.
The disease spreads upwards. Under favorable condition lesions may coalesce to become bigger lesions resulting in drying of leaves. Also we can see black dots (spores) in the centre of these lesions.
Warm, humid, cloudy weather with frequent rain favors the development of the disease. Residues of infested tobacco plants left on the soil surface are favorable for disease development.
Two applications of products containing Pyraclostrobin 40 % WG is very effective to control this pathogen. First application can be done after 20 days of transplantation and another split after 40 days.
It is a broad-spectru fungicide with protective, curative and permeation activity. Spraying can be started at the time of disease occurrence and later during the growing season and it can stop the spread of the pathogen.
Humid conditions enhance the efficacy of the fungicide. The treatment should not be started if there is less than 21 days to wait until the tobacco leaves are ready for harvest.
Always consider an integrated approach with biological products. The products based on copper oxychloride have been proved highly effective in controlling the disease