Diseases of Vegetable, Ornamental and Spice Crops

DISEASES

CLASSIFICATION OF DISORDERS:

1. Nutritional disorders: These are pre-harvested in origin and may develop as a result of deficiency of minerals in plants viz. tip burn of lettuce and blossom end rot of tomatoes are associated with the local deficiency of calcium. Boron deficiency can lead to internal black spot of beet while blotchy ripening of tomatoes is associated with a lack of potassium. For control of nutritional disorders fertilizers must be applied according to the requirement.

2. Respiratory disorders: Insufficient oxygen during storage results in anaerobic respiration leading to “black heart” of potatoes. In vegetables, non-green tissue appears to be especially susceptible to respiratory disorders e.g., cauliflower curd and the pale mid ribs of lettuce leaves.

3. Temperature disorders:

a. Heat injury: It may occur before harvest e.g., sun scorch of tomatoes.
b. Freezing injury: The cells are killed and release fluids on thawing leaving vegetables wilted and discoloured.
c. Chilling injury: It occurs between 0-15°C. Tropical products are especially susceptible e.g., cucumbers and tomatoes. The simplest way of avoiding chilling injury after harvest is to certain the critical temperature for the commodity in question and to ensure that an appropriate temperature is maintained.

4. Miscellaneous disorders: Some disorders are associated with the senescence. Nutritional factors are responsible. Ethylene can also induce specific disorders such as russet sprouting of lettuce and bitterness in carrots.
 

SOME IMPORTANT DISEASES AND THEIR SYMPTOMS

1. Anthracnose: It is a group of diseases characterized by black lesions which become covered with pink spore mass. Important hosts include bell pepper, bean and cucurbits.
2. Blight: Examples of blight which may be encountered after harvest include various fungal and bacterial blights affecting pods of peas and beans, early and late blight of potato tubers. Botrytis blight of rose and geraniums.
3. Blotch: Purple blotch of onions is caused by a fungal pathogen.
4. Canker: Canker diseases may result in infected fruits e.g., bacterial canker of tomatoes.
5. Leaf spot: In cabbage, dark leaf spot and light leaf spots are caused by unrelated fungi.
6. Scab: Scab may result from infection by filamentous bacteria (e.g., common scab of potatoes), a primitive fungus (powdery scab of potatoes) or one or many other types of fungi (cucumber scab).
7. Dry rot: It is caused by fungi, bacteria or by environmental conditions e.g., dry rot of potato by Fusarium spp.
8. Soft rot: It is caused by fungi e.g., soft rot of carrot (Rhizopus spp.) or by bacteria e.g., soft rot of potato (Erwinia spp.). Potato, tomato, chilli, brinjal, cabbage, cauliflower, radish, turnip, onion, carrot, cucumber and cut flowers are the affected hosts.
9. Vascular diseases: The disease organism interferes with water movement within the plant, eventually causing wilting and collapse e.g., bacterial wilt of potatoes. The affected plants give rise to tubers which develop brown rot of vascular ring.

FACTORS INFLUENCING POST HARVEST DETERIORATION

a) Pre-harvest factors: Cultivars vary greatly in their susceptibility to field diseases. It is desirable to breed for post harvest quality like melons with thick skin, Brussels sprout resistant to internal browning (physiological disorder). Choice of cultivars can have an important bearing on ultimate storage life.
b) Planting material: State of health of planting material is an important factor, whether true seeds, seed tubers or cuttings. Many fungi and bacteria persist on planting material and can cause disease in field or after harvest eg. Onion seeds infected with grey mould may produce crop of bulbs which produce neck rot in storage.
c) Crop husbandry: Influential cultural practices (depending upon crop) include density of planting, orientation of rows, shading, tillage, irrigation, fertilizer regimes, weed control and mulching. Nutritional sprays affect the incidence of certain post harvest disorder.
d) Harvesting and handling technique: With regard to harvest schedules, it can be advantageous to avoid mid day and afternoon picking, and to prefer the cool of the night or early morning. Harvesting by hand being the predominant method destined for fresh market. Carrots are usually transferred as promptly as possible to high humidity and storage in an effort to maintain turgidity. Leafy vegetables should be protected from exposure to sun after harvest so as to minimize moisture loss and rise in respiration rate.
e) Handling: The handling of harvested vegetables produce varies widely according to commodity. The emphasis should always be given to minimize the bruising. The harvested vegetables are tipped into gunny bags or bamboo baskets for transport to packing houses. Care or lack of it at each stage determines the amount of injury sustained. Wounds are specially likely to become infected if there is poor hygiene namely debris left in basket or gunny bags or cull piles of rotting produce in the vicinity of packing house. Another common error is the failure to renew washing water, which can become heavily charged with fungal spores and bacteria. Chlorine (sodium hydrochloride 5%) rapidly looses the its disinfectant properties in the presence of soil particles or organic debris.

MANAGEMENT

1. Physical methods: Heat treatment particularly hot water treatment is mostly employed to control post-harvest diseases. Heat treatment may kill incipient infection, leaves no chemical residues but may unfavorably alter host physiological condition.

2. Operational factors:

a. Refrigerated storage: Storage at low temperature delays the development of post-harvest diseases by prevention of ripening and directs inhibition of pathogen.
b. Controlled or modified atmosphere: It inhibits the rate of respiration of the produce and activity of decaying organisms. Chilling injury which sometimes enhances rotting can also be checked through CA storage e.g., cabbages.
c. Low pressure or hypobaric storage: Leaf vegetables retain their green colour and ripening is retarded.
d. Common storage: Storage at ambient temperature may be quite satisfactory for onions and potatoes.

3. Cultural control: It includes water management, crop rotation, nutrient management etc. Sanitation measures like removal of infested plant material, cleaning and sterilization of implants are expected to reduce primary inoculum.

4. Chemical control: Maximum research efforts have been directed towards chemical control. Large numbers of chemicals are used in different forms.

Post harvest dips in chemicals are more effective and hence most popular because of:

i) Convenience as a method,
ii) Compatibility with post-harvest practices e.g., hydro-cooling of vegetables,
iii) Total submergence for maximum penetration,
iv) Easy control of temperature and pH.

In India, the control efforts have been mainly concentrated towards checking the spoilage in more important economic crops such as tomatoes and potatoes.