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.

The spring growing conditions have been responsible for several problems that affected head lettuce in coastal California. Rain and cold temperatures have allowed significant development of bacterial leaf spot (Xanthomonas campestris pv. vitians) and a physiological disorder possibly related to tipburn. In addition, recent samples, received by UC Cooperative Extension, have been infested by an insect. Field personnel and pest control advisors have also been detecting this problem.

Underneath the wrapper leaves, inner layers are being fed upon by the larval stage (maggot) of a fly insect. Damage consists of holes and breaks in the leaves where the maggot has been feeding (photos 1, 2, 3).

Photo 1

 

Photo 2

Photo 3

Edges of the damaged areas turn tan to brown. Such feeding damage can occur anywhere along the leaf and midrib tissue. Careful examination of the inner leaves will likely reveal the presence of the maggot (photo 4) and/or the pupa (photo 5). Maggots are small (approximately 7 mm (1/10^th inch) long) and pale in color.

Photo 4

Photo 5

The insect damage, which consists of actual holes in the tissue, is distinct from the physiological problem that typically does not result in breaks in the leaf and which is usually restricted to the leaf margins. The bacterial leaf spot disease affects mostly outer leaves and results in characteristically black lesions.

Identification of the fly is pending and Entomology Farm Advisor Jianlong Bi will be investigating this further.

The rainy and cold spring weather in 2010 is apparently having an effect on head lettuce quality in Salinas Valley fields. Symptoms appear as very small, brown flecks and spots along the margins of young leaves (photo 1). Affected leaves are usually found deep within the head. It appears that these defects are occurring in multiple iceberg cultivars in various parts of the valley. Clearly this is a physiological disorder and superficially looks similar to russet spot; however, most of the flecks do not occur on the leaf midribs (photo 2) as would be typical for russet spot. Russet spot is caused by ethylene production and can occur in mature to over mature lettuce, especially following anaerobic conditions in the field. However, in this instance, the location of the flecking along the margin of the leaf more closely indicates tipburn.

The extensive nature of the problem (from Salinas to San Ardo) and the occurrence across varieties indicates that a large-scale factor like weather could be the cause. The heavy rain on April 5 followed by cloudy, cool weather may account for the currently wide distribution of the problem. We are conducting further investigations to more closely determine the cause of this problem. 

These defects are not caused by any plant pathogen. Extensive testing has shown that bacterial leaf spot, anthracnose, or other lettuce disease is not associated with these brown flecks. Bacterial leaf spot (caused by Xanthomonas campestris pv. vitians), however, is common this year and should not be confused with this physiological disorder. Bacterial leaf spot occurs on the outer leaves and results in large, black, angular lesions (photo 3).

Photo 1: Lettuce defects in 2010

Photo 2: Typical russet spotting of lettuce

Photo 3: Bacterial leaf spot of lettuce