Damping off and root rot
These diseases are caused by Pythium spp., Rhizoctonia solani, Phytopthora parasitica and a few other organisms. These are very serious diseases affecting plants at the nursery stage.
Pre-emergence damping off consisted of killing the seedlings at initial stages of seed germination and emergence. Post-emergence damping off consisted of rapid shrinking and darkening of cortical tissue of the hypocotyls. As the secondary thickening of seedlings got underway, the incidence of early post-emergence damping off declined.
The soil inhabitant fungus was generally, active at low temperatures contrary to Rhizoctonia which ramified at higher temperatures.
Control measures consisted of soil treatment with formaldehyde 2 weeks before sowing and seed treatment with 1% mercuric chloride. Spraying of seedlings with fytolan or captan was effective. Hot water seed treatment at 52˚C for 30 minutes was recommended along with 5 year crop rotation.
The disease appeared on the foliage at any stage of growth. Brown to purple black lesions began at any point on the leaflet, rachis, petiole or stem and advanced rapidly to cause a severe blight when conditions were favourable. Lesion growth was checked with low humidity. When lesion advanced to outermost zone it was pale yellow. On lower side of leaves, the white fructification of the fungus appeared where the pale and purple areas joined. In fruits, symptoms were mostly at stem end. Grey water soaked spots enlarged to indefinite size and shape.
The fungus survived between the crop seasons on other host plants and refuge like potato. Fungus also lived in soil. From potato it might be transmitted to tomato seed beds or both the crops could be attacked at the same time.
Fungicide sprays of Dithane Z-78 and partate were effective for controlling the disease. Other fungicides found effective were Dithane M-45, Difolatan, daconil 2787 (Gupta, 1988) and metalaxyl (Barrantes, 1989).
Garita et al. (1999) reported that a species of Pencillium, pencillium (067) either alone or in sequence with 2 applications of chlorothalonil was effective in reducing the disease. Extracts from bryophytes like Bazzania trilobata and Diplophyllum albicans were also found to have high efficacy against P. infestans pathogens. The antifungal metabolites from these liverworts acted as resistance inducers (Mekuria et al., 1999).
Buckeye rot (Fruit rot)
It was caused by Phytophthora parasitica and the disease appeared on lower fruits. The pathogens did not affect the foliage and thus the disease was distinct from late blight. The disease appeared as a brownish spot, often at the point of contact between the fruit and the soil. As the spot enlarged, the surface of lesion assumed a pattern of concentric rings of narrow, dark brown and wide, light brown bands.
This has wide host range and is a soil inhabitant. Plenty of soil moisture and soil temperatures between 18 and 30˚C are congenial for development of the disease.
Control measures consisted of sprays with copper fungicides and avoidance of poorly drained soils. Rattan and Saini (1969) linked association of ascorbic acid with the resistance. In susceptible conditions, ascorbic acid content ranged from 17 to 22 mg/100g. Spraying of Difolatan 4 times at 10 day intervals was effective. A few cultivars like Money-Maker, Flat Large Red, Red Cherry were resistant in the field (Sharma et al., 1974). Two sprays of copper Oxychloride 50 at 0.25% proved effective in reducing the incidence of buckeye rot (P. nicotiana var. nicotiana). Staking also reduced the incidence of the disease (Bhardwaj et al., 1995).
The early blight, one of the most common foliage blights of tomato was caused by Alternaria solani. The pathogen was first recorded in 1882 in New Jersey. Disease appeared first as spots on leaflets. They were circular to angular dark brown to black and ranged from pin head to 4 mm in diameter in size. Concentric ridges often formed leathery necrotic tissues. Usually a narrow chlorotic zone around the necrotic spot faded into normal green. Numerous spots lead to premature senescence and dropping of leaves. On the stem dark spot occurred which lead to collar rot. Fruits were infected both at the green and ripe stage. At stem end, dark black or brown lesions usually developed with sunken spots which became leathery.
The mycelium viable in dry infected leaves for more than a year. Contamination of seed might occur during seed extraction. The fungus was transmitted between successive crops through the debris and the seed. Heavy dew with frequent rains seemed to be necessary for sporulation. High soil fertility tended to reduce the severity of the infection.
In young plants, symptoms like clearing of veinlets and drooping of petioles occurred. In field, yellowing of the lower leaves appeared first and the affected leaflets wilted and died. Other leaves gradually got affected along with the vascular system resulting in stunted growth and ultimate death of the plants. Samson et al. (1942) found that it occurred rarely in seed extracted from infected plants. Chief means of dissemination was by transplants, wind borne soil and drainage water.
Control measures therefore, pertained to soil sterilization and its amendment in order to check the pathogen population. Soil solarization using polythene mulches (transparent and black) considerably reduced the pathogen population from the soil especially under irrigated conditions (Raj and Kapoor, 1993). Chemical control measures also included seed treatment with fungicides like agrocit 50 SD and chinoin-fundozol 50 WP (Iloba and Achor, 1989). Mladenov et al. (1989) recommended the use of methyl bromide and ditrapex (dichloro-propane with 1, 3- dichloropropene) as soil infectant for controlling F. oxysporum f. sp. lycopersici and other Fusarium spp. under polythene cover.
Powdery mildew is caused by Erisiphe spp., Oidium lycopersici, Sphaerotheca fuliginea, Leveillula taurica, etc. white pustules appeared on the upper surfaces of leaves of affected plants and only rarely on the under surfaces. Stems were severely affected. Correll et al. (1988) observed that powdery mildew occurred at an early stage of crop development with each successive planting date and was not related to the physiological age of the crop or any specific meteorological conditions. More lesions were observed on older leaves than younger leaves.
Biological control of powdery mildew caused by Sphaerotheca fuligiena included a combination of Bacillus brevis (bacterial biocontrol agent) and plant extracts from Reynoutria sachalinensis. The plant extract was found to act indirectly by inducing resistance in the plant (Seddon and Schmitt, 1999).
Tomato crop is attacked by a number of insect pests, some important insect pests of tomato are:
Tomato fruit worm (Heliothis armigera)
Moths are brown to yellowish brown up to 4 cm long with slight stripes. The caterpillars were greenish. Immediately after hatching, the caterpillar crawls over the leaves and feeds on vegetative parts. It eventually finds its way to the fruit into which it cuts holes and burrows. The life cycle is completed in 28-60 days. The period of activity commences from October and continues up to March.
Singh and Chakal (1978) found best results by alternate sprays with Malathion and Carbaryl at 10 day intervals starting from leaf perforation. It was found that frequent spraying of insecticides gave better control of H. zea.
Srinivasan et al. (1994) recommended T. erecta as trap crop for controlling fruit worm infestation in the field. Solanum, nigrum was observed to be heavily infested by H. armigera and was also recommended for use as a trap crop for H. armigera to prevent damage to tomato crop (Talekar et al., 1999).
Epilachna beetles (Epilachna vigintioctopunctata)
Both larvae and adult feed on the leaves. Feeding is irregular and gives the plant a characteristic lace-like appearance. Spraying of endosulfan or dichlorophos or chlorophyriphos at 0.1% controls this pest.
Jassids (Empoasca devastans)
Jassids suck the sap from the leaves causing curling of the leaves. Use of any contact insecticides like Malathion 0.05% effectively control this pest.
Tobacco caterpillar (Spodoptera littoralis)
The insects are grayish brown with white marking on upper wings. The thorax and abdomen are light brown. Young caterpillars feed on tender leaves, shoots and fruits at night (Ayyar, 1963). The pest is generally confined to nursery beds and is classed as cutworm. Their life cycle is completed in 32 to 49 days.
It could be controlled by spraying 1ml nuvan-100 mixed in 2 litres of water at the rate of about 800 l/ha. Spraying of 0.05% endosulfan is also effective.
White fly (Bemesia tabaci)
They are minute white insects and suck the sap of the plants. It acts as vector for transmitting leaf curl disease. Control measures have already been discussed in leaf curl virus disease.
Chemical control of white fly included spraying of different pesticides and pyrethroids. Spraying (with pyrethroids) and cooling of greenhouse (with fans which forced outside air into the greenhouse) resulted in reduction of population of white fly (Zipori et al., 1988).
Thrips (Thrips tabaci and Frankliniella intonsa)
White swelling spots appear on the fruit due to oviposition of thrips. Murai (1988) reported that Franliniella intonsa adults occurred from the beginning of May to end of November, numbers peaked in mid-June to end of July. He found that thrips over-wintered in a state of reproductive diapauses. Sprays of 36% DMTP wettable powder (methionine) and 50% MEP emulsifiable concentrate or soil treatment with granular formulation of 5% acephate and 5% monocrotophos effectively controlled the insect. Successful protection from thrips was achieved using UV absorbing vinyl film house.
Blossom end rot
This is a very common and destructive disorder. Lesions appear at bottom end of the fruit while it is green. Water-soaked spots appear at the point attachment of the senescent petals. It enlarges rapidly to 1cm or more in diameter. The affected portion of the fruit becomes sunken, leathery and dark coloured. If plants are grown in adequate soil moisture to produce rapid succulent growth and to promote high rate of transpiration, they are more subject to this type of disorder. This occurs due to sudden change in the rate of transpiration specially in reduced moisture. The accumulation of calcium and magnesium are important factors.
The fruits with ‘catface’ were characterized by the distortion of the blossom end of the fruit. Such fruits had ridges, furrows, indentations and blotches. It resembled blossom end rot but was distinctly difficult. Abnormal growing conditions during formation of the blossom appeared to cause distortion of growth of the cells of the pistil. As a result, the cells in the blossom end of the ovary died and turned dark to form a leathery blotch at the end of the fruit without the symptoms characteristic of blossom end rot.
As the fruit reached about two-third normal size, the outer wall continued to develop normally but remaining internal tissue growth was retarded. As a result, tomato fruits were light in weight; they lacked firmness and were partially filled. This is due to non-fertilization of ovule; embryo abortion after normal fertilization and necrosis of vascular and placental tissue after the fruit was well developed. Casual factors were high and low temperature and low soil moisture. High temperature and high soil moisture were predisposing factors.
Exposed fruits of tomato either green or nearing ripeness scalded readily during extreme heat. The tissue had blistered water-soaked appearance. Rapid desiccation lead to sunken area which usually had white or grey colour in green or yellowish in red fruits. Adegoroye et al. (1989) from Canada reported various sunscald symptoms such as the inhibition of fruit softening, production of tissue and differential ripening of unripe parts that determine the force deformation characteristics of the affected fruits. The cultivars with heavy foliage which provided greater protection from sunrays usually had the least damage.
Cracking of the surface of the fruit at the stem end was a common occurrence and often resulted in large losses. The cracks were of two kinds, one which radiated from the stem end and other develops concentrically around the shoulder of the fruit. Radial cracking was more common and caused greater losses than concentric cracking. Besides these, cuticular cracking was also often found on the skin of fruits.
Uses of resistant cultivars like Sioux, Manalucie, Crackproof were recommended. Polygenic inheritance was reported in respect of cracking.
This disorder in tomato was characterized by thick stems, short internodes and internal necrosis of the stem with more severe symptoms, the crease became deeper and usually formed a slit right through the stem. The disorder sometimes induced the abortion of stem growth and flower truss development and poor fruit set. Terabayashi et al. (1995) suggested that the high sulphate concentration in the nutrient solution caused small reductions in the P, K, Ca and Mg contents of the stem and large increase in the B content. Further, it was concluded that B prevented crease stem, though the effect of high sulphate remained unclear.