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(originally published January/February 2017)

California has been in a historic drought and the lack of water has been a major problem for agriculture especially for crops that depend on irrigation. Deficit irrigation may be used in some cropping systems as a potential water saving strategy (Goldhamer et al., 1999). The term “Deficit Irrigation” simply means irrigating at less than the full amount required by crop evapotranspiration needs. For fruiting trees such as peaches, because fruit yield and quality at harvest may not be sensitive to water stress at some developmental stages such as during the non-fruit bearing postharvest season, there is an interest in applying deficit irrigation strategies. Deficit irrigation has not been widely used due partially to the lack of effective and fast methods of monitoring plant water stress in near real-time and determining associated risks of applying deficit irrigation. When crops are managed under deficit irrigation, the margin of error in timing and amount of water application becomes smaller before causing yield losses. Monitoring the soil and plant water status is more critical for reducing risks of a crop failure or permanent damage to the trees.  However, current established techniques of monitoring the soil and plant water status such as neutron probe readings of soil water profile and pressure chamber measurements of stem water potential are labor intensive, and lack the timeliness needed for irrigation scheduling purposes.

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(originally published September/October 2016)

Shortly after hiring on as the inaugural executive director of the University of Arizona’s new Yuma Center of Excellence for Desert Agriculture, a public-private partnership devoted to applied agricultural research needed by the desert agriculture industry, Paul Brierley asked his stakeholders what they would like the Center to address first.  The answer was resounding:  Help us mitigate plant diseases!  And not just any plant disease – help us with the seemingly impossible-to-eradicate Fusarium wilt of lettuce.  And thus began the Center’s odyssey against the insidious disease that is wiping out entire fields during warm-season production of iceberg lettuce – costing the industry millions of dollars.

First step:  figure out what we already know.  No need to re-invent the wheel.  For that, Brierley turned to Dr. Mike Matheron.  Matheron is an extension plant pathologist and professor at the University of Arizona’s Yuma Agricultural Center.  When the disease first migrated to California and Arizona at the turn of the twenty-first century, Dr. Matheron and others from the U of A and UC Davis undertook research to better understand the disease and how to fight it.

6. TSW symptom fruit

(originally published July/August 2016)

Tomato spotted wilt virus is a thrips-transmitted virus that can infect many crops and weeds. In California’s Central Valley, in an important processing tomato production area, this virus disease may cause substantial economic damage. The most recognizable symptoms include fruit with oval protruding oval deformities or irregular concentric ring color patterns and this virus can kill shoots and plants, so both quality and yield are affected. The host range of this virus includes many common crops and weeds and likely survives the winter on a few weed or crop plants, but quickly amplifies on tomatoes in spring. Therefore, risk increases during the season. The virus is transmitted by thrips; primarily Western Flower Thrips, Frankliniella occidentalis in the Central San Joaquin Valley. The vector must feed on an infected plant as a nymph to be capable of transmitting the virus as an adult. Risk of loss due to TSWV can be reduced but management in high risk situations is going to depend upon several tactics. Resistance to the virus is available in both fresh market and processing tomato varieties; however, in some production areas, resistance-breaking TSWV has been reported and is likely to develop in regions where the gene is heavily relied upon. A few foliar insecticides have been shown to bring down thrips population densities and reduce incidence of TSWV symptomatic plants, but will not keep the virus down to commercially acceptable levels under high disease pressure. Under situations where there is a history of the virus, identify risk factors, which may include weed or crop sources of the virus in early spring, ensure that transplants are not arriving with infection, consider variety susceptibility, plant date, thrips management programs to calculate your risk of experiencing damage. Once there is an appreciation for your risk factors, you have capacity to mitigate disease risk by modifying any of these components that contribute to a situation in which this disease causes economic damage.

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(originally published May/June 2018)

Grapevine leafroll and red blotch disease are two virus-associated diseases that should be on the radar of all grape growers. The following article will hopefully provide you an update on these virus diseases based on our current knowledge. Summer surveying of vineyards for visual leaf symptoms is a great time to assess vineyard blocks for the presence of disease.

Grapevine Leafroll Disease

Leafroll is one of the more important virus diseases of grapevines. It occurs in every major grape growing area of the world. There are five grapevine leafroll associated viruses (GLRaVs) that are serologically distinct. These single stranded RNA viruses are placed in a family called Closteroviridae. The majority of these are grouped in the genus Ampelovirus (GLRaV-1, -3, and -4), most of the viruses in this genus have been demonstrated to be vectored by mealybugs and scale insects in vineyards. GLRav-2 is in the genus Closterovirus, and GLRaV-7 is in the genus Velarivirus, there is no known vector of these two genera.

These viruses can cause similar symptoms in infected grapevines. All the GLRaVs can be transmitted by vegetative propagation and grafting; GLRaVs in Ampelovirus can also be transmitted by the mealybugs and soft-scale insects in vineyards. GLRaV-3 is the predominant species found in most vineyards worldwide. Recent surveys in the north coast have shown 80% of symptomatic vines sampled were infected with GLRaV-3.

To further complicate matters there are variants that have been identified for given GLRaV species. For GLRaV-3 there are several distinct variants known to exist. What needs to be better understood is the significance of these GLRaV-3 variants and their interactions with other viruses when multiple infections exist in a vine. For GLRaV-2 the “Red Globe” variant is known to cause graft incompatibility when grafted onto certain rootstocks (5BB, 5C, 3309C and 1103P) resulting in the decline and death of vines.

In the post-phylloxera infestation plantings that have occurred in coastal California during the past 20 years there has been an increased incidence of grapevine leafroll disease. The use of non-certified scion material has been a major contributor to this disease increase. The other issue has been the spread of leafroll (primarily GLRaV-3) from infected vineyards to adjacent vineyards planted with California-certified stock. UC research documented the rapid spread of leafroll into a certified planting from an adjacent infected block. During the 5 years of observation the annual rate of increase in leafroll symptomatic vines was more than 10% in a Napa Valley site.