In many ways our pest and disease management of fruit tree crops are exacerbated by our cultural practices. Avocado and citrus offer some very clear demonstrations of how we manage our trees can lead to reduced pesticide use. From the beginning, our selection of rootstock and scion can help lessen pest and disease problems. In both avocado and citrus we have good rootstocks which can handle problems, such as root rot more effectively than seedling rootstocks. So it is imperative that if you know that drainage will be a problem, starting off with the right, healthy rootstock helps. Also scion selection can have a major impact, as well. For example, ‘Lamb' avocado is much less prone to persea mite than is ‘Hass'. This pest can significantly impact a spray program and planting ‘Lamb' could mean virtually no sprays for this pest. Selling the ‘Lamb' fruit is then the challenge There are similar examples in citrus where one variety is more prone to a pest or disease than another.

Irrigation is probably the most important cultural factor in managing tree disease. Over, under and improperly timed irrigations are the conditions necessary for many root diseases. The Phytophthora spp. fungi are looking for distressed root systems brought on by waterlogging and other stressful situations. Other conditions, such as wetted trunks can also bring on some trunk diseases, like gummosis in citrus and crown rot in avocado. Simply preventing irrigation water on the trunks can limit these diseases. Other diseases, such as black streak, stem blight and bacterial canker in avocado are bought on by soil moisture stress.

Nutrients, especially nitrogen management, has been long known to affect levels of insects, such as scale, mealy bug and aphid. Encouraging lush growth helps sustain these insects, so reducing this growth tends to lower their numbers. Managing when canopy growth occurs can affect pest severity. Avocado thrips build their populations in the spring and moves easily from leaf to fruit causing significant scarring. By promoting leaf growth at flowering time with a nitrogen application, keeps the insect on the leaves and reduces fruit scarring. This also promotes growth that replaces leaves that have been damaged by persea mite. Likewise the incidence of citrus leaf miner damage can be reduced if spring pruning is avoided so that a flush of growth does not occur at the same time as the population is building. Timing of pruning is important in lemons to avoid wet periods of rain and fog to reduce the spread of hyphoderma wood rot fungus when its fruiting bodies are active.

Pruning can change pest pressure by changing the humidity in the canopy, introducing light and changing the climate supporting disease and pests. By making spray coverage more thorough, it also makes for a more effective application. Modified skirt pruning can have significant effects on mealy bug and scale control, fuller rose weevil incidence, ant colonization and snail damage. It's important that the trunk be protected as an avenue of movement for snail and ant control to get the best effects of this pruning. Skirt pruning also reduces problems with such weeds as bladder pod and the ladder effect of brown rot in citrus – fungal propagules splashed from the ground onto low-hanging fruit, which in turn is splashed to higher fruit.

Keeping a canopy clean of dust and fire ash also makes for more efficient biological control. Because predators are slowed in their search, they are less efficient. They also spend more time grooming their sensory organs, and this also slows them down. Parasites such as wasps are actually slowed by the physical abrasion to their tarsi. Dust also creates a drier environment, which is more hospitable to our pest mites. Watering picking rows, roads and even the trees themselves can lessen mite populations. Use of cover crops can also reduce dust and potentially provide pollen and nectar for predators and parasites. Of course cover crops create a whole new set of management issues, such as colder winter orchards and snails.

Finally harvest timing to avoid pest and disease is often overlooked. In avocado, fruit is often set in clusters. Greenhouse thrips love the microclimate created, and if in a size-pick the cluster is reduced, greenhouse thrips will often not be a problem. Harvest timing is also important in citrus. Fruit left too long on the tree can often develop septoria fungal spot. Picking in a timely manner reduces the incidence of this disease.

These are just a few examples of how cultural practices at the right time can reduce pest and disease problems.

California Avocado Society's

2020 Annual Meeting

Annual meeting recording

Along with additional reading material

CAS 2020 Irrigation Primer Presentation

Hort 140 CIMIS irrigation calculator April-May  2020

Using CIMIS Spatial Data for Irrigation Management Avocado

Ben Faber and Brad Hanson

This is not good. You find an avocado tree with sun blotch or it is time to thin the orchard and you remove the offending tree. You know that if you don't remove the sucker, you'll end up with some rootstock growth that just gets in the way of the other trees. Avocado suckers can look like a valued tree until it's time for harvest several years later, and then you are likely to find that it's not the variety that you thought it was. Homeowners often find this problem several years after a freeze and the lemon tree that regrew from the freeze damage turns out to be the rootstock variety and produces some gnarly, seedy, juiceless fruit. Even without a frost, sometimes rootstocks which are selected for their vigor, can be more vigorous than the scion variety and will overgrow it. You then end up with whatever the rootstock fruit turns out to be.

In some situations, it is legal and common to use a “cut stump” treatment to kill stumps and prevent resprouting. In these cases, glyphosate or triclopyr is sprayed, drizzled, or painted onto a freshly cut stump. Relatively high concentrations of the herbicide are applied to the cambium, which is the living tissue just under the bark.   Cut stump treatments work well in many situations, including citrus orchards. This type of cut and spray treatment is commonly done to remove undesirable plants, like arundo and weedy tree species. However, in some trees, like avocado and many forest species, there can be root grafting, which are tree-to-tree root connections.

Due to root grafting in a mature avocado orchard, it really can be one giant root system, one tree connected to all the other trees. And if a systemic herbicide is injected in one tree, the surrounding trees can be affected – they might get enough herbicide through the root graft to be injured or even killed along with the target tree. This technique has been used in Florida to remove Laurel Wilt Disease infected avocado trees which can rapidly infect surrounding trees with the killer fungus. This is a helpful technique, because it removes any doubt that all infected trees have been killed to prevent the spread to healthy trees in the orchard.

In a healthy orchard in California, this is not a really good way to remove avocado stump sprouts. Every year reports come in of glyphosate killing good trees that surround a removed tree. The photo below is a recent case where the stump (circled in blue) was scored and painted with glyphosate. Within two weeks the surrounding tree were also killed. The systemic material was translocated from the cut surface by way of root grafts to the neighboring trees. And those trees are now dead, too.

So what to do? One thing done by those with a front-end loader or a backhoe, is to pull the stump and have an end to the sucker problem. It also reduces the possibility of chronic armillaria fungus persisting to infect trees. The problem is that it leaves a big hole to deal with which can open up a slope to erosion. If on a slope, it requires a decent sized tractor that can safely be operated on the slope without tearing up everything, including the irrigation system. And in the end, it's expensive.

The other approach is to just cut the tree down as low as possible without damaging the chain saw. Then as the irrigator makes inspections, just physically knock off the suckers as they come up. If walking the irrigation lines, it's not a problem. Covering the stump and immediate area with a physical barrier such as thick, black plastic sheet (greater than 5 ml), can reduce the number of suckers. To speed degradation of the stump, the top of the cut can be scored and a salt such as urea or magnesium sulfate (both at 10 pounds per stump) can be applied. At this rate, rather than fertilize the stump, under moist conditions, this treatment facilitates the activity of wood-decaying microorganisms; it can also damage or reduce the regrowth of the suckers.

There are also a range of registered contact herbicides that can be used to burn out the suckers. Materials, such as Scythe, Axxe and Suppress are all registered for avocado sucker control. There are others. These contact herbicide work best on small tender suckers so don't let the suckers grow more than a foot or so. For best control of suckers, apply them at the highest allowable rate with an approved adjuvant at a spray-to-wet rate. Because these products are not systemic, you'll likely need repeat applications, as new fresh buds break and new suckers erupt.

Using a contact spray means the grower would still need to be out in the orchard controlling the suckers. The grower still needs to be out in the orchard checking the irrigation lines. Why spray the suckers when they can just be broken off?

Although systemic herbicide can be used effectively to control suckers or stump sprouts in some tree crops or situations where root grafting does not occur, this is not a recommended practice for avocado because of the risk of damage to nearby trees.

The false chinch bug (FCB), Nysius raphanus (Hemiptera: Lygaeidae), is a pest of many plants. FCB is a generalist and has been found to be a problem in many cropping systems such as soybeans, quinoa, tobacco, cotton, broccoli and other Brassicaceae plants. FCB adults (above) is mostly light to dark gray, elongate, and about 0.12 inch (3 mm) long. Females lay eggs on host plants or in cracks in soil. The mostly pale gray nymphs have inconspicuous reddish to brown abdominal markings. FCB has 4-7 generations per year with all stages being potentially present throughout the year. All stages can be present throughout the year. They also can be found invading homes in the southwest. Their populations generally start in unmanaged fields with lots of weeds and are an issue for crops when they build up large numbers and move into the crops from the unmanaged, weedy fields.

Photos: Surrendra Dara

This year it's host of choice is young avocado plantings in Ventura County. False chinch bug occasionally causes severe injury on young trees by sucking sap from shoots and young stems. Infested shoots wither and die suddenly after attack, which typically occurs in May and June. Economic damage normally occurs in groves away from the coast only on young trees in border rows adjacent to uncultivated areas or grasslands. Otherwise healthy mature trees tolerate bug feeding.

Here are photos of damage to young avocado provided by Tom Roberts, Integrated Consulting Entomology.

To best manage FCB, a grower will need to catch it before it establishes and the populations explode. This is difficult because the pest will not reoccur every year on regular basis. From what has been seen in the field this year, FCB appears to prefer young avocado plantings and thus, a targeted approach is to monitor only in new plantings right as summer temperatures are rising. In paper in the journal, Phytoparasitica from 2006, the authors investigated what color sticky trap was best for monitoring and found that yellow worked best. Thus, passive monitoring with yellow sticky cards that are placed throughout the field and monitored weekly is a potential option. However, this approach can be expensive with the labor hours needed to properly process the sticky cards. A more practical approach is to sweep net weedy areas on the outside of avocado groves and adjacent unmanaged areas nearby weekly in search for the first signs of FCB.

In conventional avocado production, there is only one insecticide recommended for use against FCB. Malathion 8 at 16 oz/acre.