The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released July 12, 2011.

Area-wide incidence of whiteflies and CYSDV in desert melons

By John Palumbo, UA Research Scientist and Extension Specialist

Growers are beginning to prepare local fields for fall melons and with that comes the threat of cucurbit yellow stunting disorder virus (CYSDV).

The virus was first identified in desert melons in Fall 2006 when widespread infection on cantaloupe, honeydew, and other melons cost growers a significant portion of the crops.

USDA statistics suggest that fall melon yields in Arizona were 30 percent lower in 2008 and 2009 compared with previous years. This yield reduction is likely due in part to the impact of CYSDV.

Over the past five years, we have been studying the virus and trying to understand its epidemiology and impact on fall melon production. In addition, we continue to develop new information to control the CYSDV vector (Bemisia whitefly adults).

Whitefly numbers this spring and summer have been relatively light, and the incidence of CYSDV on spring melons was relatively low. Given the aggressive management programs that pest control advisers and growers are now using, it will be interesting to see how CYSDV impacts melons this fall.

Our research suggests that the area-wide incidence of CYSDV may be light to moderate this fall. However, based on what we don't know about the virus-vector relationship, an accurate prediction of CYSDV this fall is not possible.

To view a summarized report of our research, click on this link - http://extension.arizona.edu/sites/extension.arizona.edu/files/resourcefile/resource/marcop/Areawide%20CYSDV%20-%202011%20summary_Veg%20IPM%20Update02.pdf

Contact Palumbo: (928) 928-782-3836 or jpalumbo@ag.arizona.edu.

Summer preplant soil flooding - a management tool for sclerotinia lettuce drop

By Mike Matheron, UA Extension Plant Pathologist

Lettuce disease management is probably the last thing on PCA and growers’ minds as we enter the hottest part of the year in the Desert Southwest. However, this is the perfect time to perform preplant soil flooding in fields that had high levels of Sclerotinia drop this past season.

How can soil flooding help manage a disease that will not be a problem for several more months?

The two fungi that cause lettuce drop, Sclerotinia minor and Sclerotinia sclerotiorum, carry over in fields between crops of lettuce as structures called sclerotia. These fungal propagules function like seeds remaining dormant until germination in cool, moist soil and infect lettuce plants.

Many sclerotia in soil decay naturally over time. However, sufficient numbers can remain in a field after one or more years to cause lettuce drop when a planting is established. If virtually all sclerotia in a field could be destroyed, then this field would no longer be a source of the Sclerotinia lettuce drop pathogens.

This is where summer preplant soil flooding comes in. Past research conducted at the University of Arizona’s Yuma Agricultural Center demonstrated that a three-week period of flooding in the summer destroyed all sclerotia of S. minor and S. sclerotiorum present in soil. Some growers in the Yuma area have used this procedure to help manage lettuce drop in fields chronically affected by this disease.

For more information, review this research paper - Plant Disease Vol. 89, No. 1.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.

Sprangletop

By Barry Tickes, UA Area Agriculture Agent

Sprangletop has become increasingly widespread in Arizona mostly because of its growth habits and tolerance to many commonly used herbicides.

Sprangletop is in the Leptochloa genus which is derived from the Greek words leptos (thin) and chloa (grass). There are more than 150 species of sprangletop worldwide but only three in Arizona and two in Yuma County.

The two that are the most common in the low desert are Mexican Sprangletop, Leptochloa uninervia, and Red Sprangletop, Leptochloa filiformis. A third species, Bearded Sprangletop, Leptochloa fascicularis, is more common at elevations of 1,500 feet or higher.

It is common to find Red and Mexican Sprangletop in the same field and it is not hard to distinguish them when they are side by side.

Red Sprangletop has a light-green leaf blade which is similar in width to watergrass and barnyardgrass. It has very fine hairs and very small and fine branches and spiklets. It also has a long membranous ligule. Red refers to the leaf sheath which is characteristically red, rather than the seed head.

Mexican Sprangletop has a thinner leaf blade which is darker green or grayish in color and similar in appearance to common bermudagrass. The seed head is distinctly coarser than Red Sprangletop.

Side by side, leaf color and seed size make it easy to distinguish the two. Both grasses are classified as summer annuals but grow more like perennials in the low desert.

Sprangletop does very well in the hottest part of the summer and typically germinates from seed during the hottest period between July and September. Once established, it often survives through the cold winter months.

It grows into clumps that often appear dead during the winter. New shoots commonly grow from the established crowns the next season. When this occurs, pre-emergent herbicides including Trifluralin or Prowl are ineffective. Some Sprangletop plants stay green and grow through the winter.

Many post-emergence, grass-specific herbicides that control many grasses are ineffective on Sprangletop. This also has contributed to the spread of these weeds.

Sethoxydim (Poast) and Fluazifop (Fusilade) do not control Red and Mexican sprangletop. Only Clethodim (Select Max, Select, Arrow, and others) is the only one of these grass herbicides that is effective and only at the highest labeled rates. Two applications are often necessary to achieve season long control.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu.

Question of the Week

Insect management: what level of whitefly pressure can growers and PCAs expect this fall?

A - light populations

B - heavy populations.

C - Ask John (video)