Pest & Disease Management

 

Pest and Disease Management:
Weeds: Most growers use only mechanical cultivation and hand-hoeing for weed control in the growing regions. Herbicides are applied with shallow incorporation, and transplants are placed with the roots below the treated zone. Postemergence herbicides are used to control grasses. Metam sodium is used for preplant weed suppression.  Plastic mulch provides excellent weed control over the plant row. The inter-row space can be kept clean by using a registered knockdown herbicide. Where crops are grown without plastic mulch, grass control is obtained by applying registered post emergence herbicides (It is important to read the chemical label, melons are very susceptible to herbicide damage especially when the plants are small). Mechanical methods will give good control and are most effective if the vines are “tucked in” before cultivation. Cultivation between rows should be shallow to avoid injury to the root system.

 

Insect pests of watermelon: A number of insect species including caterpillars, mites and thrips can cause damage to the plants but are readily controlled using registered insecticides. Aphids can be very damaging as they are the vector for mosaic viruses.

 

Scientific Name Common Name Damage Control
 Aphis gossypii  Melon or Cotton Aphid  Sucks sap and weakens plant.Leaves curl, shrivel and may turn brown and die Secretion of ‘honeydew’ causes sootymould to develop.  Biological – use of ladybird beetle Chemical – Fastac Decis, karate (1.5ml/ L water) or Sevin 85 wp (1.5g/ L)
 Diaphania hyalinata  Pickleworm or melon worm  Caterpillars feed on leaves and flowers. Sometimes they                bore into the developing fruit.  Use of appropriate contact insecticide such as malathion (2 ml/l of water)
 Diabrotica seporata  Leaf cutting beetle  Developing seedlings are affected by adults Leaves become (2 ml/l of water) skeletonized.  Use of appropriate contact insecticide such as malathion
 Thrips palni       Thrips  Sucks sap of leaves which become scorched. Infestation or most severe during sunny weather  1. Crop rotation  2. Chemical –Regent (10- 30 ml/L water), Vydate L (2.5 ml/ L water) or Admire
 Bemisia tabaci  White fly  Sucks plant sap, leaves become chlorotic and are shedded prematurely  Cultural – keep farm free of weeds Chemical – Use soap based products or other chemicals such as Pegasus or Vydate l at the recommended rates.
 Liriomyza sp.  Leaf miner  Larva feeds      between leaf surfaces resulting in ‘chinese writing”  Chemical control – using Trigaul, Admire, Pilarking, Vertimex, Abamectin and Newmectin at the recommended rates.

 

Disease Management:
Diseases that affect watermelons are similar to those of pumpkins. Diseases can cause extensive damage to crops. The following are some common diseases of watermelon:

 

Disease Symptom Management
 Bacterial fruit blotch  Oval water-soaked areas on fruit  crop  *Registered seed treatment, hygiene, *registered copper-based sprays
 Fusarium wilt  Yellowing, wilting, stunting  Rotation and resistant varieties
 Gummy stem blight  Brown spots on leaves; reddish ooze from runners; black sunken spots on fruit   *Registered fungicides and crop hygiene            
 Mosaic viruses (CMV, PRSV-W, WMV, ZYMV)  Mosaic pattern on leaf, leaf and fruit distortion   Deter aphids, crop hygiene
 Powdery mildew  Greyish patches on older leaves  *Registered fungicides or resistant strains
 Damping off Root-knot nematodesSudden death  Rotting and death of seedlings Wilting and stunted growthRapid wilting Rotting roots  Good soil drainage Soil treatmentImprove drainage control irrigation crop rotation.

 


Advanced stage of lesion development of bacterial fruit blotch of watermelon
Leaf symptoms of bacterial fruit blotch

 

Vertebrates: Mice can cause major problems in melon crops prior to emergence; they will dig out and eat large quantities of seed. The crops may have to be replanted several times, resulting in delayed harvests. Pre-germinating seed or planting container grown seedlings will reduce loss.

 

Crows can be a devastating and annoying pest. Just before harvest they can reduce the salability of melons by punching holes through the skin with their sharp beaks. Often not content with eating from just one melon, the crow will hop from melon to melon, in this way populations of crows can wipe out an entire crop in just a few days. Crows can be deterred with the use of a sound device such as a gas gun, or bird alarm.

 

Fruit physiological disorders:
Physiological disorders are caused by nonpathogenic agents that affect fruit quality. Usually, aesthetic qual­ity is degraded. The cause can be either one or a combination of environmental, genetic, or nutritional factors.

 

Blossom-end rot (BER) is caused by uneven irriga­tion that leads Ca-deficiency in the flower (blossom) end of the fruit. It is worse in hot, dry, windy conditions where moisture stress is more likely to occur. Symptoms include young fruit drop and brown rotting lesions at the blossom end of older fruit. Good water management and ensuring sufficient soil calcium availability will usually address the problem. Soil or irrigation water salinity may also promote blossom end rot.

 

Blossom-end rot results in dark-brown, sunken leathery lesion on the blossom-end of fruit.

 

Internal cracking is caused by cool temperatures during early fruit-filling period. Other influences are stop-start growth, excess nitrogen, low boron levels, or heavy infrequent watering at fruit fill. Affected melons tend to be flattened in shape and feel lighter than usual.

 

 
Hollow heart fruit is expressed as flesh separation
Bottleneck is caused by poor pollination that results in constricted stem-end growth

 

Sunburn can be a major problem in dark or darker striped melons. It is rarely seen in light colored melons. Sun burning can also occur in melons stacked in the field with their underside facing upward.

 

Cross stitch is the name given to this disorder by Dr. Don Maynard of the University of Florida. It is characterized by elliptical or oval lesions that tend to be positioned perpendicular to vascular bundles on watermelon rind (See picture below). The disorder occurs very infrequently. Cross stitch was observed in Indiana in the late 1980s and early 1990s. Attempts to isolate a pathogenic organism from affected tissue were not successful and confirmed initial suspicions that cross stitch is a non-infectious disorder

Sunburn results in destroyed pigment in the rind
Cross stitch is expressed as necrotic lesions or cracks that are perpendicular to longitudinal axis of fruit.
 
Greasy spot causes circular, raised, olive-green areas on surface of rind		  
Target cluster is the result of target-like on the rind configurations

 

Spongy end occurs in poorly pollinated melons. These melons may turn yellow and drop off the vine early in their development or partly develop with the stylar end soft and spongy. This area is also slightly pointed. Internally, there is very poor seed development at the spongy end.

 

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