Two important virus pathogens have been affecting coastal lettuce crops for a number of years. As expected, both problems have shown up again this summer of 2010. These virus diseases are familiar to experienced growers and pest control advisors. However, one should exercise caution in diagnosing these problems because their respective symptoms can resemble each other.

Impatiens necrotic spot virus (INSV): INSV-infected plants have leaves with brown to dark brown spots and dead (necrotic) areas; this necrotic tissue can resemble burn damage caused by pesticide or fertilizer applications. Extensive necrosis can cause much of the leaf to become brown, dry, and dead. Some leaf yellowing can also be observed. Yellowing and the brown spotting tend to be observed on the newer leaves near the center of the plant’s growing point. If plants are affected with INSV early in their development, growth may be stunted. All lettuce types are susceptible, and INSV has been confirmed on iceberg, butterhead, romaine, and leaf lettuces. INSV can infect many other crops and weed species; the virus is vectored by thrips.

Lettuce necrotic stunt virus (LNSV) and Tomato bushy stunt virus (TBSV): Diseased lettuce can be severely stunted, especially if infected early in plant development. The oldest, outer leaves can be severely yellowed. Brown, necrotic spots and lesions later develop in these outer leaves. The younger, inner leaves remain dark green in color, but can be rough and leathery. LNSV/TBSV infects only romaine, butterhead, and leaf lettuces; modern cultivars of iceberg lettuce are immune. The LNSV/TBSV virus complex is a soilborne problem. No vector (insect, nematode, fungus) is known to spread these viruses.

See table below for comparison of these virus disease symptoms. For help in diagnosing these and other plant problems, submit samples to the UC Cooperative Extension diagnostic lab in Salinas.

Symptom comparisons for INSV and LNSV/TBSV pathogens of lettuce
Symptom		INSV	LNSV/TBSV
Presence of yellowing, chlorosis		yes	yes
Yellowing mostly on older leaves		no	yes
Brown necrotic spots, lesions		yes	yes
Stunting if infected early		yes	yes
Central part of plant remains green		no	yes
Affects romaine and leaf lettuce		yes	yes
Affects iceberg head lettuce		yes	no

Impatiens necrotic spot virus (INSV) on lettuce

Lettuce necrotic stunt virus (LNSV) on lettuce.

In 2008 we received a 24C for use of Dual Magnum on spinach; however, two issues make the current version of the label difficult to use: 1) the plant back interval for lettuce is 12 months which seriously limits its utility in Salinas Valley rotations, and 2) the preharvest interval (PHI) is 50 days. The plant back issue is still not resolved; however progress was recently made on the PHI.

The IR4 program conducted residue studies to change the PHI to 21 days. We just received news about the residue trials: Dual Magnum residues collected at this interval exceeded the current tolerance and a 21 day PHI will not be possible. As a result, Syngenta is currently trying to settle for a PHI of 40 days. This would be an improvement over the current PHI of 50 days, but still leaves Dual Magnum in a grey zone for use on clipped spinach which commonly matures in 30 days or less.

One alternative that we have explored is the application of Dual Magnum prior to planting. The question that comes up is do you lose a certain percentage of the Dual Magnum if it is applied to the top of the bed and it sits for a period of time before being incorporated into the soil with sprinkler irrigation following planting. In San Ardo in 2009 we conducted a trial in which Dual Magnum was applied to shaped beds on July 1 and the field was planted on July 21. We observed good efficacy at the 0.75 and 1.0 pint rates (Table 1). There was little efficacy at the 0.50 pint/A rate when the Dual Magnum remained on the soil surface for 21 days prior to planting; normally we see reasonable weed control at the 0.50 pint/A rate when Dual Magnum is applied immediately following planting and incorporated into the soil with sprinkler irrigation. It therefore appears that Dual Magnum can remain on the soil surface for at least 20 days, but higher rates may need to be used to obtain weed control equivalent to what is needed for at-planting applications.  We will need further evaluations of this application technique to better understand the rates and timing. 

Table 1. Weed counts and phytotoxicity rating on August 6, 2009

Treatment	Material Per Acre	Lbs a.i./A	Purslane	Malva	Other weeds	Total weeds	Phyto
Dual Magnum	0.50 pint	0.48	41.3	0.8	1.3	43.3	0.0
Dual Magnum	0.75 pint	0.72	4.0	1.8	2.3	8.0	0.8
Dual Magnum	1.00 pint	0.96	1.0	2.8	4.3	8.0	1.3
Untreated	----	----	3.0	11.8	21.8	36.5	0.0
Pr>Treat			<0.001	<0.001	<0.001	0.002	0.005
LSD 0.05			16.4	3.7	5.7	17.0	0.7

 

In spring and summer of 2010, plants in some tomato and pepper fields in coastal California have developed virus-like symptoms. Affected plants were found in both Monterey and San Benito counties, though disease incidence was low. Samples submitted to the Gilbertson lab at UC Davis tested positive for the curly top virus, indicating that these symptoms were due to curly top disease. 

Curly top can be a severe disease for both tomato and pepper. Symptoms of the disease vary depending on the host plant infected and the stage of growth when infections occur. In general, affected plants show some degree of stunted and distorted growth and leaf curling and light green to yellow discoloration. In tomato, curly top symptoms begin with leaves showing light green coloration, up-curling, and vein purpling on the underside of leaves. Plants become stunted with twisted, distorted, and yellow leaves. Plants infected at an early stage of development stop growing and die, often standing out among nearby healthy plants. Plants infected at later stages of development show distorted growth and light green-yellow leaves with vein purpling; these symptoms will develop on the upper part of the plant or on entire shoots, but the plants generally do not die. Any fruits produced will be stunted and ripen prematurely.

In peppers, plants are stunted with shortened internodes. Leaves are light green or yellow in color, show strong up-curling, and are thick and brittle. Plants infected at a young stage of growth may die, whereas older plants are stunted and leaves again curl upwards and are light green to yellow; any fruits that develop are small, wrinkled, and unmarketable. As for all diseases caused by viruses, confirmation of curly top disease requires laboratory testing because other tomato and pepper viruses can cause symptoms that are similar to those caused by the curly top viruses (e.g., symptoms caused by Tomato spotted wilt virus). 

In California curly top disease affects many plants including tomato, pepper, bean, pumpkin, squash, spinach, sugar beet, other crops, and many weeds. Researchers have discovered that curly top disease in the western USA is actually caused by several distinct virus species. Therefore, curly top of tomato or pepper could be caused by one or more of the following viruses: Beet curly top virus (BCTV), Beet mild curly top virus (BMCTV), Beet severe curly top virus (BSCTV). For example, samples in 2008 from Monterey County tested positive for either BMCTV or BSCTV. 

The curly top viruses are vectored by the beet leafhopper (Circulifer tenellus). The virus is not carried in seeds nor is it mechanically transmitted (i.e., by touch or physical contact). Disease incidence and distribution in tomato and pepper fields in a given year are dependent on the populations of the beet leafhopper and the migratory feeding patterns of this insect. Most infections tend to occur early in the growing season, but late season infections may also occur, as in 2010. In general, for coastal California, disease incidence is usually low and symptomatic plants occur randomly in a field, indicating where the fast moving leafhopper has stopped to feed. The random, scattered distribution of diseased plants reflects the fact that tomato and pepper are not preferred hosts of the beet leafhopper and that the insects move on in search of preferred hosts, such as plants in the beet family (Chenopodiaceae).

Tomato infected with curly top viruses: If infected early, plants can be severely stunted.




Tomato infected with curly top viruses: Leaves can be thickened, deformed, yellowed, and rolled upwards. Symptomatic tomato leaves often develop purple veins.




Pepper infected with curly top viruses: If infected early, plants can be severely stunted.




Pepper infected with curly top viruses: Leaves can be thickened, deformed, and rolled upwards.

In the months of July and August of 2010, a number of vegetable crops in California’s central coast were affected by calcium deficiency disorders. Usually referred to as “tipburn” (except in the case of celery, for which the disorder is called “blackheart”), these problems are often triggered by environmental factors and hence can affect a large number of acres and plantings during windows of time when tipburn is occurring in Salinas and other coastal valleys.

Tipburn symptoms on lettuce, endive, radicchio, and spinach are distictive. Symptoms occur on the margins of developing leaf tips and initially consist of light to dark brown lesions and necrosis. In romaine, tipburn is often first seen on the small veins along the margin of young leaves. In severe cases tipburn can progress and result in extensive damage to these leaf margins. Symptomatic leaves are usually found within the inner whorls of open head vegetables and underneath the enclosing wrapper leaves of closed head vegetable types. For spinach, tipburn always affects the inner, newly developing leaves which may develop a hooded appearance as the leaf continue to expand around the dead tissue. For blackheart of celery, symptoms form on the margins of developing leaf tips deep within the central growing point. Such symptoms consist of light to dark brown speckling, lesions, and necrosis. As the celery grows, the damaged tissues turn black and the affected foliage grows up and out of the inner plant whorl.

For tipburn of artichoke, the margins of immature leaves turn black as in celery. In addition, the immature artichoke flower buds develop black lesions along the upper tips and edges of flower bracts. Cauliflower also develops tipburn, with the inner wrapper leaves enclosing the cauliflower head turning tan to light brown.

Tipburn is a localized calcium deficiency. It often develops along the margins of leaves in the final weeks before harvest when the plant growth rate is at its highest. It is often unrelated to soil calcium levels. Because calcium is not very mobile within the plant, expanding tissues on young leaves and growing points may run short of this essential element and begin to develop deficiency symptoms. Conditions that favor rapid plant growth (warm temperatures and high fertilization rates) or low transpiration rates (foggy conditions) may create conditions that trigger this disorder.

Managing this disorder is difficult. Varieties vary in susceptibility, but tipburn resistance is only available in head lettuce. Maintaining even soil moisture levels and preventing the root zone from drying out helps increase calcium uptake and may reduce the risk or severity of tipburn. For field grown vegetables, soil and foliar calcium supplements have not provided consistent control.

For further information on tipburn see Monterey County Crop Notes - July/August 2007

Initial symptoms of tipburn on romaine occur on veins at the edge of the leaf.



Tipburn of butter lettuce.




Tipburn of cauliflower.




Blackheart of celery.



Tipburn of artichoke.