The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released June 14, 2011.

Sanitation and whitefly-CYSDV management

By John Palumbo, UA Research Scientist and Extension Specialist

With the spring melon harvest well under way, it is important to begin thinking about whitefly management in fall produce and melons crops.

The first line of defense in avoiding whitefly issues in fall plantings is for pest control advisers and growers to do a good job controlling whiteflies on cotton this summer. This may be particularly important since the cotton acreage has increased significantly this year.

However, before whitefly management begins in cotton, it is important that whitefly populations be prevented from building up to large numbers in the spring melons that are currently being harvested.

In surveying melon crops for Cucurbit Yellow Stunting Disorder Virus (CYSDV) this spring, it became readily apparent that a large proportion of the spring melon acreage throughout the area was grown near cotton.

In fact, our surveys show that on an area-wide basis more than 83 percent of the melon acreage this spring was grown either adjacent to or within a one-quarter mile of cotton (See Melon Survey). In the Dome Valley and Wellton, about 95 percent of the melon acreage was grown adjacent to cotton.

Although whitefly numbers have been relatively light thus far, increased whitefly numbers have been observed over the past week in melons coinciding with higher temperatures and area-wide melon harvests.

Proper sanitation in spring melons is critical to preventing unnecessary whitefly buildups in cotton. It is highly recommended that melon growers quickly destroy plant residue as soon as possible following harvest. A delay in disking under melon fields following harvest can provide a large source of adult whiteflies that will readily disperse into cotton especially when they don't need to fly very far.

These fields also potentially extend the host-acquisition period for CYSDV. Good news though - to date CYSDV incidence in spring melons has been relatively low this spring, and was most evident in the north Yuma Valley as shown in the 2011 Spring Melon CYSDV Survey.

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

Pythium linked to sudden wilt and death of watermelon plants

Pythium linked to sudden wilt and death of watermelon plants

By Mike Matheron, UA Extension Plant Pathologist

In past years, fungal diseases including Fusarium wilt, Charcoal rot, and Monosporascus root rot and vine decline have been implicated in the loss of melon plants this time of year in southwestern Arizona and southeastern California.

This year, Pythium, a fungus-like plant pathogen, has been associated with significant losses of plants in several desert watermelon plantings.

Several different Pythium species are found in agricultural fields. Pythium species can colonize melon feeder roots at any time. However, lateral root and taproot infections, which can lead to plant collapse and death, usually are observed following irrigations after fruit set.

Whether or not Pythium infections will proceed to cause melon plant damage or death is dependent on environmental factors including soil temperature and moisture, plus plant stress.

With respect to temperature, some Pythium species can cause disease at low to moderate soil temperatures whereas others are most active at higher temperature levels. All Pythium species thrive in very wet soil as water is essential for production and spread of the pathogen spores.

Cultural factors that trigger disease development include fruit load and late season water deficit stresses.

In California greenhouse studies, melon plants growing in the presence of Pythium displayed symptoms of sudden wilt when the plants had set fruit and were subjected to a brief water stress period followed by flooding for one or two days.

On the other hand, symptoms of sudden wilt did not occur when plants were maintained in a fruitless condition or were not subjected to a water stress period.

Sudden wilt is occurring this year in many watermelon fields where healthy-appearing plants suddenly collapse during the heat of the day; followed by permanent wilting and death a few days later. The onset and severity of sudden wilt will vary from field to field with the speed of plant collapse associated with the degree and rate of root infection.

Management considerations for sudden wilt caused by Pythium include careful irrigation water management and the application of an effective fungicide including mefenoxam (Ridomil Gold) at the first appearance of disease symptoms.

Remember that other diseases, including Fusarium wilt, Charcoal rot, and Monosporascus root rot and vine decline mentioned earlier, can cause similar symptoms. However, mefenoxam will have no effect on the fungal pathogens that cause these diseases.

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

The effect of fire on weed seeds

The effect of fire on weed seeds

By Barry tickes, UA Area Agriculture Agent

Vegetation management with fire is a very old technique used in agriculture. It dates back to prehistoric times but is still used today. Fire is a part of nature and is necessary to maintain many grassland and forest ecosystems.

Fire is used in this region primarily as a post-harvest technique to remove dead or dormant plant material. It is most commonly used in the summer after grain is harvested to facilitate ground preparation for the next crop or in the spring prior to the regrowth of bermudagrass to remove stubble and stimulate regrowth.

One of the benefits of burning is it can be effective in killing weed seeds. The effect of fire on weed seeds has been the subject of many studies and is dependent on many variables. These include the weed type and species, the depth of the seed in the soil, the heat of the fire, and the duration of the fire.

The type and species of weeds found most affected by burning are summer annuals. Weeds that produce seeds with hard seed coats are the least affected. These include sweet clovers, mallow, dodder, sesbania, and others.

Fire has little effect on creeping perennials especially during the dormancy period. These include bermudagrass, field bindweed, perennial johnsongrass and nutsedge; all of which have below ground reproductive structures.

In weeds that have become established, seedling grasses are more sensitive to fire than seedling broadleaves.

This is primarily because the growing points of many grasses are below the soil while the growing points of broadleaves are above the soil and unprotected. Seeds that are burned before they become viable are also more sensitive to fire.

The depth of weed seed in the soil also will affect sensitivity to fire. In one study, 88 percent of the seed located in the top 1 millimeter (mm) of soil was killed while only 18 percent of the seed buried 5 mm (0.19”) into the soil was killed. Tillage following burning can also bring up viable seed.

The effect of temperature on seed viability has also been the subject of many studies. In most, the germination of summer annuals increased with a rising temperature to about 140 degrees F. and dropped off rapidly as temperatures approached 212 degrees F. Imbibed seeds were also more sensitive to fire than un-imbibed seeds.

In addition to temperature, the duration of the fire has been found to affect seed mortality. Hot, slow fires kill many more weed seeds to greater depths than light flash fires that raise the temperature only in the top layer of soil.

At one experimental site, wheat stubble with longer straw burned longer and hotter than short stubble.

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