Post harvest Management of Horticultural Crops (2+1)

Methods of disinfestation

Methods of disinfestation
A. Field management: Growing fruits and vegetable adopting scientific standard and recommended practices can reduce the field inoculum of disease causing pathogens. Adopting standard cultural practices in terms of sanitation, proper nutrition, irrigation and appropriate harvesting time and methods, etc. are known to reduce post harvest diseases. Use of wind breaks can reduce spread of field infection where wind is the carrier of pathogen. Other practices like cultivation of crops in regions free from diseases, cultivation of disease resistant cultivars, care in harvesting and handling to avoid wounding fruit, fruit bagging for reducing surface wetness and deposition of inoculum will all help in reducing post harvest diseases.

B. Pre-harvest spraying: Field sprays with fungicides are known to prevent spore
germination and the formation of deep seated infections in the lenticels or in the floral remains of the fruits.

C. Post harvest chemical treatments: Post harvest treatments with fungicides like Thiobendazole and Benomyl have rendered good control of stem end rot in many citrus fruits, anthracnose of banana and mangoes despite the fact that infection occurred long before the treatment was applied.
Safer and less toxic chemicals grouped under the category of GRAS (generally regarded as safe) can be used for the control of post harvest diseases of fruit and vegetables. These compounds mostly include week organic acids, inorganic salts and neutralized compounds. It has been reported that extracts of Eucalyptus globula, Punica granatum, Lawsonia inumis, Datura stramonium and Ocimum sp. extracts are effective against various fruits rots. Some vegetable and other oils are also effective against fruit rots. Mustard, castor and paraffin oils have been found effective against Rhizopus rot of mango.
Disinfection of all handling equipment in pack-houses with 1-3% formaldehyde solution, hypochlorite or SOPP (Sodium ortho phenylphenate) will help in prevention of secondary infection. Washing with water alone reduces many disease of fruits and vegetables.

Methods of Chemical Application
1. Dipping – for effective control of diseases chemical may be used with hot water at 550C
for about 10 min. The crop may be passed below shower of the diluted chemicals. This is called ‘cascade’ application. Use of chemical like citric acid to lower the pH of the solution along with fungicides seems more effective.
In pineapple, infection occur commonly through the cut fruit stalk, therefore dipping cut end was found sufficient to control the disease, save pesticides solution and lower residues on the fruits.
Eg. Citrus, apples, pineapple, root vegetables.

2. Spraying - Spraying is more effective than dipping, because fungicide effectiveness is
reduced if the crop has been washed and is still wet and many pesticide chemicals are formulated so that they are not in a solution , but rather in a fine suspension. This results in a concentration gradient in the tank between top(less concentration) and bottom (more) of the tank unless suspension frequently agitated.
Eg. Citrus, apple

3. Electrostatic Sprays / Thin film of Coating – breaking up the pesticides solution into fine
droplet and then giving them an electric charge to obtain uniformity of application. Principle is that the particle all have the same electrical charges hence, thus repel each other. These charges are attracted toward the crop and form uniform coating on the produce.
Eg.: Potato and crown rot of banana

4. Dusting – with wood ash and lime in case of yam. Fungicides along with talc on potato.

5. Fumigation / Vapour treatment – Fumigation is to eliminate insects, either adults, eggs, larvae or pupae and pathogen inoculum. Fumigant such as sulphur dioxide (SO2) is used for controlling post harvest disease in grapes. This is achieved by placing the boxes of fruit in a gastight room and introducing the gas from the cylinder to the appropriate concentration. This treatment results in a residue of 5-18 ppm SO2 in the grapes is sufficient to control decay. Its toxicity to Botrytis cinerea was found to be proportional to temperature over the range of 0-300C, where the toxicity of SO2 ¬increased about 1 ½ times for every 100C rise in temperature. In general treatment with 0.5- 1% SO2 for 20 min is found to be effective fallowed by ventilation. During storage, periodic (every 7-10 days) fumigations are performed in concentrations of 0.25%.

Disadvantages

• SO2 can be corrosive, especially to metals, because it combines with atmospheric moisture to form sulphurous acids. Hence, special sodium met bisulphate impregnated pads are available which can be packed into individual boxes of fruits to gives a slow releases of SO2. Eg.: Grape guard used in grapes fruit packing.
• At higher concentration it has bleaching effects on black grapes.
• Some people are allergic to SO2, particularly those who have chronic respiratory problems.


Litchi fruits - SO2 fumigation at 1.2% for 10 min. is used to prevent discolouration of the skins of fresh litchi fruits caused by fungal infection, followed by 2 min. dip in 1 N HCL stabilizes the red colour and reduces the skin browning.
Snap beans - Exposing the beans to SO2 at 0.7% for 30 seconds reduced the broken end dis- coloration due to mechanical injury.

Other chemicals

• Acetaldehydes fumigation in Sultana grapes @500 ppm for 24hr. control postharvest diseases.
• Paper pad impregnated with diphenyl fungicides are commonly applied to citrus fruit.
• Tecnzane, 2-aminobutane(potato) and 2-AB (orange) are the chemicals used.


Fumigation with gaseous sterilants is the most effective techniques for disinfesting produce. However, these are becoming increasingly unpopular or banned because of high mammalian toxicity (hydrogen disulphide), flammability (carbon disulphide) and damage to the atmospheric ozone layer (methyl bromide).

Fumigation with methyl bromide has been replaced by temperature (high and low) treatments, controlled atmosphere, other fumigants or irradiation.


6. Absorbent paper – chemical may be absorbed into a pad made of suitable material like paper. This absorbent pad soaked in fungicides like thiabendazole and dried, is placed over cut surfaces, such as cut crown end to control the crown end rot of banana. Here pad absorbs latex from the cut surfaces, which also helps to keep the pad in the position and prevents staining the banana. Potassium aluminum sulphate may be added to the pads, which helps to coagulate the latex. This method is used when banana is dehanded in the field and packed directly into export cartoon, where no washing, spraying or dipping take place. Insecticides like dichlorovos has limited vapour phase activity, therefore dichlorovos based pest strip have been included in cartoon packed with flowers to effects ongoing disinfestations during export.

7. Cold storage – many insect pests do not tolerate prolonged exposure to low temperature.
Storing the produce at <1.60C for 16 days has been shown to be effective for disinfesting fruits against Mediterranean and Queensland fruit fly. But chilling susceptible fruits are not suitable for the this method

7. High temperature – Heat treatments like hot water dips or exposure to hot air or vapor is
employed for insect control (and for fungi, in some cases). Using high temperature of about 40-550C for about 15 minutes can be easily disinfected. Generally, high temperatures can cause softening of tissues and promote bacterial diseases.


• Dipping temperature depends on commodity, insect to be controlled and its degree of development.
• Dipping in hot water also contributes to reduced microbial load in plums, peaches, papaya, cantaloupes, sweet potato and tomato but does not always guarantee good insect control.
• Heat treatments is reconsidered as quarantine treatments in fruits such as mango, papaya, citrus, bananas, carambola and vegetables like pepper, eggplant, tomato, cucumber and zucchinis.
• Temperature, exposure and application methods are commodity specific and must be carried out precisely in order to avoid heat injuries, particularly in highly perishable crops. On completion of treatment, it is important to reduce temperature to recommended levels for storage and/or transport.
• Many tropical crops are exposed to hot and humid air (40-50 °C up to 8 hours) or water vapor to reach a pulp temperature which is lethal to insects. Hot air is well tolerated by mango, grapefruit, Navel oranges, carambola, persimmon and papaya. Similarly, vapor treatments have been used for grapefruits, oranges, mango, pepper, eggplant, papaya, pineapple, tomatoes and zucchinis.
• A common mango fruits disease, anthracnose can be successfully controlled by dipping at 550C for about 5 min.

8. Biological control –

• The yeast Candida guilliermondii is used against Penicillium spp. incorporated into citrus waxes
• Bacillus subtilis is used against mango anthracnose and stem end rot