Watermelons - Common Diseases and Physiological disorders


Watermelons are susceptible to several diseases that attack the roots, foliage, and fruit.
Disease control is essential in the production of high-quality watermelons. A preventative program that combines the use of cultural practices, genetic resistance, and chemical control as needed usually provides the best results. Cultural practices are useful for limiting the establishment, spread, and survival of pathogens that cause watermelon diseases. Many of the fungal, bacterial, and nematode pathogens survive in old crop debris and in the soil. Fields with the proper soil characteristics should be selected.

Disease

Verticillium Wilt

Symptoms

Yellowing of the foliage and wilting.

Control

Copper oxychloride 85 WP

Disease

Anthracnose

Symptoms

Early signs of infection by this bacterial disease are water-soaked spots on the leaves and fruit.

Control

Mancozeb, Chlorothalonil.

Disease

Fusarium wilt

Symptoms

Affects runners of the plant, at advanced stage may cause wilting

Control

Benomyl 50 WP

Disease

Powdery mildew

Symptoms

Premature defoliation, and reduced crop growth.

Control

Wettable sulphur, Bavistin, Benomyl 50 WP.

Disease

Charcoal rot

Symptoms

Death of crown leaves combined with grey stem lesions.

Control

Apply potassium and phosphorous fertilisers

Disease

Downey mildew

Symptoms

Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface.

Control

Polyram DF, Copper Oxychloride 85 WP

Disease

Rhizoctonia and soil-borne diseases.

Symptoms

“Damping off”, or the failure of infected seeds to germinate.

Control

Seed dressings with Vitavax Plus or Thiram 80 WP.

Fruit Pruning

Fruit pruning in watermelons should begin as soon as defective melons are noticed.
Remove misshapen and blossom end rot fruit to promote additional fruit set and better
size of the remaining melons. If a market demands larger melons, remove all but two or
three well-shaped melons from each plant. To avoid disease spread, do not prune
melons when vines are wet.

Physiological disorders

Physiological disorders are caused by non-pathogen agents that affect fruit quality. Usually, aesthetic quality is negatively affected. The cause can be either one or a combination of environmental, genetic or nutritional factors. Below is a list of some of the common fruit disorders:

Misshapen melons (gourd-necked or bottlenecked) are frequently produced by varieties with long fruit. Moisture stress is a cause. Occasionally melons of any variety may misshape because they lie on uneven ground or were damaged when small. Misshapen or pear-shaped fruit can also be caused by poor pollination that leads to restricted growth at the stem end because of the absence of developing seeds. Poor pollination can be minimised by increasing the number of beehives in the field. Low temperatures can also cause misshapen fruit.

Blossom end rot is a deterioration of the blossom end of the fruit. The usual order of development is softening, slight shrivelling, browning, blackening with extensive shrivelling, and sometimes secondary decay. Poor calcium nutrition and moisture stress cause blossom-end rot. Hot, dry winds, nematode damage, excessive fertiliser, low levels of calcium in the soil, pruned roots from late cultivations, and other conditions are contributing factors.

Bursting may result from an uneven growth rate, which is particularly associated with heavy rainfall or irrigation when the fruit is maturing. The percentage of bursted fruit is usually low, and types with round fruit are more susceptible.

White heart consists of white streaks or bands of undesirable flesh in the heart (centre) of the fruit. This is caused by excessive moisture (and too much nitrogen) during fruit maturation.

Hollow heart is a disorder that varies among varieties. Hollow heart is marked by cracks in the heart of the water melon fruit owing to accelerated growth in response to ideal growth conditions facilitated by ample water and warm temperatures.

Sun Scald (burn) results from exposure to intense solar radiation that leads to dehydration and overheating damage of the rind tissue. Sun Scald can be alleviated by covering the fruit with vines or straw materials.

Sun burn occurs most frequently in varieties that have dark green rinds. Charleston Grey types and other melons with grey grin rinds rarely suffer from sunburn. Good healthy foliage will minimise sunburn damage as well as favour good yields and quality. Strong winds can blow unprotected vines away from the developing fruit along the edges of the rows and cause full exposure of the fruit to the sun.

Rind necrosis is an internal disorder of the watermelon rind. Symptoms are brown, corky, or mealy textured spots on the rind which may enlarge to form large bands of discoloration that rarely extend into the flesh. Experienced pickers often can detect affected melons by the subtle knobbiness that is visible on the surface of the affected melons.

The cause of rind necrosis is unknown. Bacterial infection has been reported to be a cause, although similar bacteria are found in healthy melons. Drought stress also is reported to predispose melons to rind necrosis.

Cross stitches are elongated necrotic wounds (2 cm long) that are perpendicular to fruit length. The cause of cross stitches is unknown.

 

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