- Author: Larry J Bettiga
Pinot leaf curl (PLC) is a physiological disorder that affects the Pinot cultivars. The symptoms are general seen in the coastal production areas during cool spring temperatures. During the cool spring temperatures of 2010 symptoms of this disorder were very common to Pinot vineyards. This week I have had several reports of this disorder showing in Pinot vineyards in the central coast.
Symptomatic leaves on elongating shoots curl downward across the middle of the leaf blade, perpendicular to the mid-vein. The angle of the downward bend is acute; the marginal tip of the center lobe may come into contact with the petiole. Mild symptoms involve only the blades; continued expansion of which results in a misshaped and reduced leaf size. A necrotic region is present on the mid-vein preventing further elongation. If the necrosis expands to include the petiole, the leaf will abscise from the shoot. Severe symptoms occur when necrosis involves the node at which a leaf abscised, killing the shoot distal to that point.
Symptoms appear somewhat similar to Botrytis shoot blight caused by Botrytis cinerea on foliage under cool, wet spring conditions. In the past samples of PLC affected tissue have been tested and no known foliar pathogens have been associated with the samples. Fungicide applications are unwarranted and will have no effect on subsequent PLC symptom development.
Research conducted by Rhonda Smith, UCCE Sonoma County and Doug Adams, Department of Viticulture and Enology UC Davis, have shown the level of putrescine were found to be elevated in symptomatic versus asymptomatic samples. Relative amino acid levels were less clearly aligned with symptoms; symptomatic blades were associated with elevated gluatamine levels in addition to elevated putrescine. Glutamine is the predominate amino acid in vine sap and the main amino transport form of reduced nitrogen in the vine. With the current information, pinot leaf curl is a physiological disorder most likely associated with elevated nitrogen levels. Elevated putrescine levels are toxic to plant tissue and are thought to be involved in symptom development in false potassium deficiency (“spring fever”) and early bunch stem necrosis. Pinot leaf curl may be an additional disorder associated with putrescine.
Source: Adapted from UCCE Sonoma County (http://cesonoma.ucanr.edu/viticulture717/Viticulture_Publications/)
- Author: Steven T. Koike
As early as March 2015, leafy green crops in the Salinas Valley exhibited symptoms indicative of virus diseases. In fact, viruses in the tospovirus group have been confirmed in a number of fields in the Valley:
1. Impatiens necrotic spot virus (INSV) on lettuce. Diseased leaves have tan, brown, or blackish spots and dead sections; this necrotic tissue can resemble damage caused by pesticide or fertilizer applications (see photos 1 and 2 below). Some leaf yellowing can also be observed. Symptoms can be found on both the older, outer leaves as well as on the newer leaves near the center of the plant growing point. If plants are affected early in their development, growth can be stunted. A number of fields have been confirmed to be positive for INSV, though in each case disease incidence was low (well below 1%).
2. Tomato spotted wilt virus (TSWV) on radicchio. Diseased leaves have yellow spots, streaks, mottles, and other symptoms (see photos 3 and 4 below). Brown necrotic leaf spots are sometimes present. If radicchio is infected when young, the plants are severely stunted, leaves are deformed and twisted, and head-forming cultivars fail to develop normal heads. Disease incidence was moderately high.
The INSV and TSWV pathogens are spread by thrips. In our coastal region, these tospoviruses are spread only by the western flower thrips (Frankliniella occidentalis). In the early part of 2015, growers and PCAs reported moderate to high thrips pressure already. This abundance of the vector certainly accounts for these early outbreaks of tospovirus disease. A number of weeds and other hosts serve as reservoirs of the viruses. Immature thrips feed on these infected plants, develop into adults, and can then spread INSV and/or TSWV to lettuce and other surrounding host crops. The dry spring is contributing to early drying up and senescing of weeds and hillside vegetation; this early decline of surrounding vegetation could be driving thrips populations into fields earlier than normal. Radicchio and all lettuce types (iceberg, romaine, greenleaf, redleaf, butter) are susceptible to both TSWV and INSV, so field personnel should be aware of this early development.
Plants suspected of being infected with tospoviruses can be sent for analysis to the UC Cooperative Extension diagnostic laboratory in Salinas.
Photo 1. Lettuce infected with Impatiens necrotic spot virus.
Photo 2. Lettuce infected with Impatiens necrotic spot virus.
Photo 3. Radicchio plant on right infected with Tomato spotted wilt virus.
Photo 4. Radicchio leaves on right infected with Tomato spotted wilt virus.
- Author: Lennis Arriaga
- Author: Larry J Bettiga
Nitrogen fertilizer can have negative impacts on lettuce roots via salt effects and specific ion toxicity from nitrite and ammonium. In the early spring we commonly see ammonium toxicity in coastal lettuce production fields. In the past week, we received numerous samples of lettuce plants with this disorder. The core of the root of affected plants turns reddish brown to black, tissue becomes soft (necrotic), and the root becomes hollow as the plants age (Photo 1). The tops of affected plants suffer from water stress and become stunted; plants often wilt during the day due to the impaired root system (Photo 2). Lettuce plants with damage from ammonium may remain alive but will be stunted and behind in development; if the damage is severe and/or the plants are small, the lettuce can die. A key characteristic of ammonium toxicity is the distribution pattern in the field. Plants affected with this problem will occur randomly scattered throughout the field and will rarely occur in patches or groups.
While ammonium toxicity symptoms may resemble those of some lettuce diseases, in most cases ammonium toxicity is readily differentiated from fungal or bacterial problems of lettuce roots (Table 1). The main disease that may be confused with ammonium toxicity is Fusarium wilt. However, Fusarium wilt is characterized by occurring later in the summer, causing symptoms on both younger and older lettuce plants, occurring in patches and groups of plants, and thus far being found primarily in the southern part of Monterey County.
Table 1. Comparison of various characteristics of ammonium toxicity and soilborne diseases of lettuce
|Characteristic||Ammonium toxicity||Fusarium wilt||Verticillium wilt||Corky root|
|Stunted young plants||Yes||Yes||
|Root vascular discoloration||Yes: red to brown to black||Yes: usually red to brown||Yes: usually black||
|Root exterior surface||White, healthy||White, healthy||White, healthy||Yellow to brown, cracked|
|Affects mature plants||
|Field distribution||Random, scattered||In patches, groups||In patches, groups||May be uniform|
|Time of year||Early spring, sometimes June||Summer through fall||Summer through fall||Spring through fall|
The factors that contribute to the buildup of toxic levels of free ammonia and/or ammonium in the soil include fertilizer type, as well as soil type, temperature and moisture content. For instance, urea fertilizers results in the buildup of ammonium resulting from hydrolysis in the presence of the urease enzyme:
If the pH of the zone where urea hydrolysis is taking place increases to above 8.3 (e.g. in the fertilizer bead), highly toxic amounts of free ammonia can temporarily form. High levels of ammonium in lettuce tissue can also be toxic. In the early spring when soil temperatures are less than 60 °F, nitrification of ammonium to nitrate is slowed and ammonium can build up in the soil following the application of urea or ammonium fertilizers. We have observed soil ammonium levels ranging up to 8 ppm or more in the winter and early spring, whereas in the summer it is rare to see soil ammonium levels exceed 1.0 to 1.5 ppm. Given the sudden onset of a number of fields with ammonium toxicity, it seems clear that weather and/or cool soil temperatures created conditions favorable to the buildup of toxic levels of ammonium in the soil.
Table 2 shows data from a 2001 on-farm evaluation where calcium nitrate was compared with the standard fertilizer program of ammonium nitrate to determine the effect on ammonium toxicity symptoms. In trial 1 we did see a significant reduction in the number of heads per plot with ammonium toxicity; however, there were still some toxicity symptoms even with the use of calcium nitrate, indicating that the form of fertilizer was important, but did not answer all the questions regarding the disorder in these trials.
In coastal California ammonium toxicity is typically an issue that occurs during the months of February and March. As the days lengthen and soil temperatures increase, issue with ammonium toxicity will decrease.
Table 2. Number of heads/plot with toxicity symptoms and levels of ammonium and nitrate in the soil 2 weeks following fertilizer application
Heads withtoxicity No.
|Trial No. 1|
|Calcium nitrate||18 a||0.79 a||61.1|
|Ammonium nitrate||61 b||3.43 b||82.6|
|Trial No. 2|