Pest and diseases affecting lettuce in Trinidad

The following are a list of pest and diseases affecting lettuce plants in Trinidad, with a brief description, symptoms associated and control methods. Pictures were taken by lettuce farmers located in Orange Grove, Trinidad.

Name of pest: Root knot nematode 

Scientific Name: Meloidogyne hapla

Image 1

Illustration of galls/ swelling of roots in lettuce plants.

Description

Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They survive in the soil and plant tissues and several species may occur in a field. Symptoms of nematode infestation also vary according to the nematode species and crop type, and are often non-specific. Root knot nematode species, however, cause typical galling on roots of infested plants.

Symptoms

Infestations may occur without resulting in obvious above ground symptoms, such as stunted growth, uneven growth or delayed maturity. 

Root knot nematodes feed within the roots and cause characteristic swelling or thickening of roots (galls).

Control

Cultural

• Use resistant varieties.
• Rotavate or plow soil under infested plants to prevent further reproduction of nematodes.

Sanitation

• Clean farm equipment thoroughly with water, removing all soil as to prevent further infestation or spread. Also ensure irrigation from infested field is prevented from entering other fields. Prevent animals grazing from infested fields to uninfected fields.

Chemical

• Vydate L

Name of pest: Leaf miner

Scientific name: liriomyza sativae

Image 2

Illustration of tunneling in lettuce leaves due to leaf miners.

Description

Leaf miners are the larvae of small black to gray flies with yellow markings. Females puncture leaves to feed on plant sap and lay eggs within the leaf tissues. After 2 to 4 days eggs hatch and larvae feed between the upper and lower surface of the leaves. Larvae emerge from the mines and pupate on the leaf surface or, more commonly, in cracks in the soil. 

Symptoms/ damage

Distinctive winding, whitish tunnels or mines that are often the first clue that leaf miners are present. 

Control

Biological

• Natural enemies such as parasitic wasps can be used to help control leaf miner populations.

Cultural

• Avoid planting near infested fields.

Insecticides:

• Alverde
• Cure
• Padan

Name of disease: Bacteria leaf spot

Causative agent: Xanthomonas campestris 

Image 3

Illustration showing lesions due to bacterial leaf spot.

Description

Bacterial leaf spot can occur on both leaf and head lettuce varieties. As with most bacterial diseases, the pathogen, Xanthomonas campestris is highly dependent on wet, cool conditions for infection and disease development. Symptoms develop only if rain or sprinkler irrigation is present. 

Symptoms

An early symptom of bacterial leaf spot is small (less than 0.25 inch in diameter), water-soaked leaf spots on the older leaves of the plant. These lesions are typically bordered by leaf veins and angular in shape. Lesions quickly turn black (a diagnostic characteristic of this disease). If the disease is severe, numerous lesions may coalesce, resulting in the collapse of the leaf. Older lesions dry up and become papery in texture, but retain the black color. Lesions rarely develop on newly developing leaves.

Control

Herbicides

• Coback
• Serenade
• Cuproneb

Name of disease: Cercospora leaf spot

Causative agent: Cercospora lactucae-sativae

Image 4

 Illustration of Cercospora leaf spot.

Description/ Symptoms

Leaf spots ranging from colours tan to brown, circular or oval, up to 1 cm diameter, with small whitish centres, sometimes with yellow halos, and joining together to cover large areas of the leaf, The older leaves are infected first. Spread is due to spores produced on both sides of the leafspots, splashed by rain, and blown in wind-driven rain. These spores survive in crop debris, and also on weeds. High temperatures (25-35°C) and high humidity or rain favors outbreaks of the disease.

Control

Herbicides

• Bellis
• Carbendazim
• Polyram 80WG

Name of pest: Giant African Snail  

Scientific name: Achatina fulica

Image 5

Illustration of Giant African Snail feeding on lettuce plants.

Description

The giant African snail is nocturnal, but may become active at dawn and dusk if it is very wet. They prefer damp, shady places and avoid direct sunlight. In the day they are commonly found under ledges, bricks, rocks, logs, plant mats, decaying leaves, or discarded containers; and in or on plants, trees, heavy vegetation, brick holes, crevices.

Control

Control and eradication is achieved through quarantine, cultural and chemical means. Eradication measures must be continued for 2-4 years after last snail sighting; and monitoring for a further two years to ensure snail free status.

Preventative means include: inspecting and ensuring materials are free from snails: plants, vehicles, machinery; cessation of movement of soil/debris from snail infested areas to non-infested areas; and prohibition of rearing/moving live snails. 

Cultural control methods include, sanitation, which involves:

• destruction of snail habitats by clearing underbrush, leaves, rotting or fallen fruit/vegetables,eliminating refuse piles and other debris
• removal of loose boards and boulders 
• capturing and killing snails by crushing or drowning in boiling water or salt water, or rubbing alcohol or ethanol.

Sanitation is a continual process. Chemical treatment will include the application of recommended molluscicides to the soil (within the drip line of food plants and resting places) 200 yards beyond the nearest infested property. 

The main groups of molluscicides are: 

• Metaldehyde – which is the most widely used active ingredient in snail baits. Metaldehyde baits come in many formulations with various attractant systems. Baits containing 4% metaldehyde are significantly more effective than those products containing only 2%. Metaldehyde products formulated with carbaryl should not be used as they are toxic to soil-inhabiting beneficial organisms, such as ground beetles and earthworms. Examples of methaldehyde available in Trinidad are Bio Slug & Snail Killer, a 3% Bayer pellet bait.

• Methiocarb - This is a carbamate chemical. Methiocarb produces better kill than metaldehyde under wet conditions; Iron Phosphate – This is considered safe for the environment, pets and other non-target animals. Snails ingest the bait, cease feeding and die 3-6 days later.

References

Ezez. 2017. "Bacterial Leaf Spot / Lettuce / Agriculture: Pest Management Guidelines / UC Statewide IPM Program (UC IPM)". Www2.ipm.ucanr.edu. https://www2.ipm.ucanr.edu/agriculture/lettuce/bacterial-leaf-spot/.

"Fact sheet - Lettuce Cercospora leaf spot (210)". Pestnet.org. http://www.pestnet.org/fact_sheets/lettuce_cercospora_leaf_spot_210.htm.

"The Giant African Snail - Ministry of Agriculture, Land and Fisheries". Ministry of Agriculture, Land and Fisheries. https://agriculture.gov.tt/publications/pest-alerts/giant-african-snail/.

Mou, Beiquan & Liu, Yong-Biao. 2003. "Leafminer Resistance in Lettuce". HortScience 38 (4): 570-572. American Society for Horticultural Science. doi:10.21273/hortsci.38.4.570.

"Nematodes On Lettuce – How To Treat Lettuce With Nematodes". Gardening Know How. https://www.gardeningknowhow.com/edible/vegetables/lettuce/lettuce-with-nematodes.htm.