- 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: Richard Smith
- Author: Steven T. Koike
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 |
No |
Yes, sometimes |
Root vascular discoloration | Yes: red to brown to black | Yes: usually red to brown | Yes: usually black |
No |
Root exterior surface | White, healthy | White, healthy | White, healthy | Yellow to brown, cracked |
Affects mature plants |
No |
Yes | Yes | Yes |
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:
CO(NH2)2 | + | 2H20 | + | H+ | ---------> | 2NH4+ | + | HCO3- |
Urea | water | acid | urease | ammonium | bicarbonate |
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
Fertilizer Type |
Heads with toxicity No. |
NH4-N ppm |
NO3-N ppm |
Trial No. 1 | |||
Calcium nitrate | 18 a | 0.79 a | 61.1 |
Ammonium nitrate | 61 b | 3.43 b | 82.6 |
Trial No. 2 | |||
Calcium nitrate | 10 | 0.74 | 29.8 |
Ammonium nitrate | 24 | 0.82 | 35.7 |
- Author: Michael D Cahn
CropManage Workshop: Hands-on training
Monterey County Agricultural Center Conference Room
1432 Abbott St, Salinas CA 93901
Thursday, April 2nd 2015
(8:30 am – 12 pm)
We will offer a hands-on training to learn in depth about the features of CropManage, a free online decision support tool for water and nutrient management of coastal crops. In addition to head and romaine lettuce, CropManage now supports broccoli, cauliflower, cabbage, and strawberries.
Considering that the drought is continuing into a 4th year, and nutrient management continues to be linked to water quality regulations, efficiently using water and nitrogen fertilizer is a high priority for Central Coast growers. CropManage can play an important role in providing quick decision support on water and nutrient management on a field-by-field basis.
This training will provide an opportunity to learn how to use CropManage for improving the efficiency of your farming operations or for adding value to your consulting services. We will provide in depth hands-on training so that you can learn step-by-step how to navigate and use CropManage for assisting with fertilizer and water management decisions and record keeping. Wi-Fi internet access is available at our conference room so please bring a laptop or tablet computer so that you can follow along as we tour through the features of the software. There should be sufficient time to answer questions as we cover the following topics:
Agenda:
8:30 – 9:00 Registration and Refreshments
9:00 – 9:30 Introduction and update on CropManage
9:30-10:15 Getting started with CropManage
10:15 (Break)
10:30 – 11:15 Strategies for using CropManage for decision support and record keeping
11:15- 11:45 Advanced features and interfacing sensors with CropManage
11:45-12:00 Discussion of new features or changes needed.
To keep the group size manageable so that we can provide individual help, we would like to limit the workshop to 30 participants. If you have attended previous workshops and or feel proficient in using the on-line tool, then you are welcome to just attend the second half of the workshop (10:30-12 pm). Whether or not you plan to attend the entire or part of the workshop, please RSVP in advance by sending an email to larriaga@ucdavis.edu or mdcahn@ucdavis.edu with the subject heading “CropManage workshop” and let us know the number of participants in your group. We will email you a confirmation. Thank you, and I hope to see you soon.
Respectfully,
Michael Cahn, Irrigation and Water Resources Advisor
Certified Crop Adviser CEU hours requested (1.5 hrs irrigation management, 1.5 hrs nutrient management)