What are the 4 main types (scientific classification) of lettuce grown in the US?
- 1. Crisphead
- 2. Cos
- 3. Butterhead
- 4. Leaf
Which two types of lettuce store and ship well?
What are the names of the two sub-types of butterhead lettuce?
Is lettuce a warm or cool season crop?
How does the length of growing season (days to harvest) for heading lettuce (Iceberg and Romaine types) compare to that of leaf lettuce?
- The climatic requirements of head lettuce are much more critical than for leaf lettuce
- Crisphead: 70-80 days midsummer or 120 for late fall/winter
- Leaf: 65-80 days midsummer or 130 for late fall/winter
What does it mean to say that lettuce seeds are subject to thermodormancy?
When it is consistently approximately 85 degrees the seed produces hormones and its goes into dormancy and does not germinate.
What is currently considered to be the temperature threshold of lettuce for this phenomenon?
Imbibed seeds become thermodormant at temperatures above 85 degrees F.
Irrigation is often used to facilitate and insure good lettuce seed germination during hot weather. Explain how this is done. How does wetting the soil insure good germination? How long must one irrigate, i.e., what is the relationship between length of lettuce seed imbibition and the seeds' sensitivity to thermodormancy?
- Themodormancy can be broken by starting the initial irrigation in the late afternoon so the seed can imbibe water and germinate during the cooler hours of the night.
- Water excessively to keep soil cool
- 1st day water 12 hours
- 2nd day water 8 hours
- 3rd day water 4 hours
What is seed priming?
- Priming allows the seed to overcome thermodormancy and germinate at higher temperature.
- Primed seed is commonly used in low desert production areas.
What is osmosis?
Movement of water from higher concentration to lower causing seed to slowly absorb water
What is an osmoticum?
- Polyethylene glycol
- Seed drinks up water so put seed in osmoticum for osmosis to occur
How does seed priming affect lettuce seed’s sensitivity to high temperature?
Seed priming allows the seed to germinate at higher temperatures
How is the young lettuce plant affected by frost? The mature lettuce plant?
- At temps near freezing young plants are not damaged but growth is slow
- Freezing can damage the outer leaves of mature lettuce, leading to decay in handling and storage
What is bolting?
When plants grow quickly, stop flowering and set seeds
What day temperatures induce lettuce to bolt?
High temperatures favor bolting
How does daylength affect the tendency to bolt?
Long days favor bolting especially if seedlings previously subjected to cold temperatures
Describe the root system of lettuce, i.e., is it a shallow, moderately deep or deeply-rooted crop? How does lettuce's rooting depth affect cultivation, fertilization and irrigation? How does transplanting affect the morphology of the root system?
- Lettuce plants form a deep penetrating tap root.
- However, the major absorbing root area is found spreading laterally in the upper 12 inches of soil (considered a shallow rooting crop)
- Transplanting prevents the production of the tap root for most dicots that are transplanted.
- Transplanted lettuce will have a primarily fibrous root system.
How does overwatering affect the ability of roots to absorb water? Why?
Roots cannot move water properly so there is a build up of calcium which causes tip burn
Approximately 70% of lettuce growth occurs during what period prior to harvest? How would this sudden increase in plant size affect fertilizer usage and timing of application? Considering this growth pattern, what advantages does fertilizing through drip lines give in promoting lettuce yield when compared to conventional fertilizing programs in which lettuce is sidedressed at or soon after thinning?
75% of head growth occurs in 2-3 weeks prior to harvest
In the Salinas Valley, there is a lettuce host-free period from Dec. 7 to Dec. 21. What exactly does this mean? Why is this done?
- There cannot be any lettuce in the ground.
- They are trying to break the population of the green peach aphid. There will be no food for the aphid to eat.
- The aphid spreads Lettuce Mosaic Virus
Tipburn is a physiological disorder of lettuce generally considered to be induced by a deficiency of what mineral element? What weather conditions promote its development? Why does over or under-watering also promote its development?
- Calcium deficiency
- When the fog rolls in and there is high humidity are the best weather conditions
- Roots don't work properly and plants cannot move water so calcium builds up
The Salinas and Pajaro Valleys of Monterey County have been designated host-free areas for the Lettuce Root Aphid. What tree ornamental, the growth or planting of which, was declared illegal in these areas? Why was this plant banned?
What is LMV? How is it transmitted? Is it seedborne? What percentage of seed is infected when harvested from an infected plant?
- LMV causes the leaves of young infected plants to roll out and down.
- Followed by pale green to yellowish mosaic pattern in the young leaves
- Is seedborne
- vectored by the green peach aphid and other aphids
- 3-8% of seed is infected
Lettuce seed is tested (indexed) for LMV. Describe the procedures for the MT and MT0 methods of indexing (testing) the seed for the virus. Which method must be used for seed utilized in the Salinas Valley?
- MT (Mosaic tested): 1,000 seeds looked at chance of having virus is 0.1%
- MTO (Mosaic tested to zero): 30,000 seeds chance of having virus is approx. 0
Corky root is a disease of lettuce caused by what general class of organism? What field conditions favor its development? What is the most effective means of control? How does nitrogen fertilization affect its development, i.e., the susceptibility of the plants to infection?
- caused by bacterium Corynebacterium group
- promoted by warm weather when the roots become brittle.
- Best line of defense is to purchase resistant cultivars
- makes white tap root yellow. Reduces absorbing ability and stunts growth
Downy mildew is a disease of lettuce caused by what general class of organism? What weather conditions promote its development?
Humid, wet conditions that are cool. The spores that land on the leaves require 8 hours of moisture on the leaves to germinate and infect the lettuce tissue. Measuring leaf surface moisture (dew) is a method utilized in an Integrated Pest Management system for control of downy mildew of lettuce. By spraying only when at least 8 hours of free water are on the leaves, the amount of fungicide that must be used to control the disease can be greatly decreased.
Describe the formation of downy mildew lesions on lettuce leaves -- are they on the top, bottom, or both sides of the leaves? How is this disease normally controlled, i.e., what is the first strategy and, if this fails, what is the second?
What is vertical resistance? Horizontal resistance? Which type do farmers most prefer if only yield is considered? Which would they prefer if only pest management were considered? What is the main disadvantage to using vertical resistance? What is the main disadvantage to using horizontal resistance, i. e., what is the main disadvantage when developing horizontal resistance? Be able to draw a figure illustrating the relationship between vertical or horizontal resistance and yield over time.
What is Sclerotinia drop? What conditions favor its occurrence?
What is “big vein,” i.e., what are the symptoms of this lettuce problem? What causes it? How is it controlled, if at all?
- a virus that is transmitted by soil fungus
- areas around the veins are clear, causing the veins to appear enlarged
- surviving plants are smaller, quality of heads is affected and maturity is delayed.
The Lettuce Aphid (Nasanovia ribis-nigri) is a relatively new pest in the Salinas Valley. Why is the occurrence of this aphid so particularly bothersome? What method must be used to control it?
- Aphids that crawl into the grounds and attack aphids by sucking on roots.
- Crawl between the leaves deep into the lettuce heads where they are difficult to wash away.
At what temperature should lettuce be stored? At what relative humidity?
- 32 degrees F with 98-100% RH
- 31.7 degrees is freezing point so normally set around 34 degrees for fridge errors.
What is Russet Spotting of lettuce? What causes it? At what temperatures are the symptoms expressed? How can it be prevented or avoided?
- Olive brown spots appear on the lower midribs of outer leaves.
- Develops lesions (ovals) on head red (Russet) color
- Arises from exposure to ethylene gas
- Can be from diesel exhaust
- will promote ripening and can cause tissues to age until death.
What is Brown Stain of lettuce? What causes it? How can it be avoided?
What are the general properties of mulches, i.e., if you use them, what benefits, in general, can you expect?
- 1. May suppress weeds (not all do)
- 2. Reduces water loss
- 3. Reduces leaching
- 4. Prevents or reduces soil crusting
- -Insufficient oxygen and plant will not grow
- 5. May reduce disease
- 6. May reduce insect problems
- 7. May increase earliness of harvest
- 8. May increase size and quality of harvest
What is an organic mulch? How do organic mulches affect soil temperatures? How does organic mulch breakdown aid soil tilth?
- Made from straw or other organic material
- Organic mulches do not increase soil temperature.
How does a black plastic mulch affect soil temperature? Weed growth? When applying a black plastic mulch, how must the mulch be applied, i.e., what will increase the effectiveness of this mulch in heating the soil?
How does a clear plastic mulch affect soil temperature? How does the clear mulch heat the soil, i.e., what is the effect called? How does using a clear mulch affect weed growth?
heating soil early in season
What is the primary benefit of using a white/silver mulch in your fields? Why is this effect important?
Sometimes used with crops to repel certain insects, especially aphids
What is an "IRT" mulch. How does it work, exactly? How is soil temperature affected? Weed growth?
Infrared [radiation] transmissible or IRT mulch
Why are some investigators testing mulches with colors such as blue, yellow and red? How do these mulches act to increase crop growth and yield?
Reflect specific wavelengths of light into plant canopy to increase photosynthetic efficiency
What are common problems with using plastic mulches?
- Cost of mulch
- Special equipment required to lay plastic
- With plastic mulches, sidedressing may be a problem
- In some cases, all fertilizer may have to be in bed before plastic laid
What is a row cover? What are the types of row cover?
- Row covers are a covering over the plant
- come in floating row covers
What is the greenhouse affect? What type of row cover employs the greenhouse effect to heat the air around plants? By how many degrees can this row cover heat the air?
- A plastic cover creates the greenhouse effect. In a greenhouse UV light passes through and being readmitted though inferred which hits the water on the sides and bounces back to the plant.
- Is about 15-20 degrees warmer than the outside.
What are the advantages of using a row cover? Disadvantages?
- With row tunnels, have to carefully monitor temperatures as can get too hot and can harm crop.
What is a hot cap? Why is it used? Is it commonly used? What is an obvious disadvantage in using hot caps (other than cost).
- plastic cups that goes over a plant. Can also be a square shape(wire with plastic)
- can be used as frost protection
- Top of plant will die but part in hot cap will survive
- Waxed paper pyramids used to cover crops to give a few degrees’ frost protection
- Very labor intensive to place --> backbreaking work!
What is, "solarization"? How does one "solarize" a soil?
- Used in areas of high solar radiation to partially sterilize soil
- Reduces some plant diseases and weed problems