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.  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 practice at the right time can reduce pest and disease problems.

Stem and leaf blights are symptoms that appear for various reasons – high rainfall or humidity, spray burn, chewing insect infestation.  Here in California we can add other causes, such as drought and salinity burn.  These conditions can cause wounding of leaf and stems allowing entry of fungal spores that can cause leaf and stem dieback.  This condition is most common near the coast where weather conditions can change from mild and low temperatures to extremely high temperature with winds, such as the Santa Anas or the Sundowners in Santa Barbara.  Leaves suddenly dry out, causing cracking either at that time or when they are rehydrated with irrigation.  This allows spore entry into the wounds and permits the pathogen to grow in the dead tissue.  Symptoms appear 7 – 10 days after the stress.  These are decay fungi that create these spores and they are the ones that cause decay of dead tissue on the ground.  So their spores are everywhere. 

The greater part of a tree is dead – the woody part of the branches and trunk.  And it is dead tissue that these fungi are feeding on. Most trees will limit the growth of the fungus by sealing off the infection with gums of various sorts.  In that case, the disease is limited and you may only see a leaf or small branch dying back. In mature trees it is possible to see a small branch here and there that has died back, but the bulk of the canopy is still green.  It has been called “salt and pepper syndrome”, because of that speckled appearance.  In the case of young trees with their smaller root systems and a lesser ability to seal of the disease process, a whole tree can die.

Since this is a severe water stress or salt stress induced problem, the most important management issue is to watch the weather forecasts predicting unusual hot, dry weather and make sure the trees are adequately irrigated going into the stressful period. Shallow rooted trees like avocados are more prone to dry out rapidly in these high water demand situations, but it can be occur in other trees (citrus, apple, peach) and shrubs if the weather conditions are severe enough. With poor leaching due to low rainfall, this can be more of a problem

The only solution to the symptoms is to cut out the diseased parts to prevent its further spread. Once the disease starts spreading, the fungus can produce copious amounts of spores, which in the case of avocado can cause cankers and rots on the fruit.

Figure.  In the case of young trees, the whole tree may die from blight.

At a recent conference on Postharvest Technology Advances, Cristina Davis from the UC Davis Department of Mechanical and Aerospace Engineering presented information on the development of a device that can smell out trees infected with Huanglongbing (HLB).

Scientists at UC Davis are refining a mobile chemical sensor that can detect diseased citrus trees by sniffing their volatile organic compounds (VOCs). VOCs are emitted by all types of plants and contribute to their distinctive odors—such as the perfume of orange blossoms and pungent scent of garlic in the air. VOCs must exist at very high levels for humans to smell them, and there are some VOCs people cannot smell at all.  The machine is able to figure out the signature smells of HLB infected trees, sort of the way people evaluate wines with terms like “grassy”, “plum”, or “austere”, and distinguish them from  healthy or trees infected by other diseases.

Finding HLB-infected trees and eliminating them before Asian Citrus Psyllid (ACP) picks up the disease and spreads it to neighboring trees is a major challenge. The pathogen in the tree cannot be detected by leaf testing for three to nine months after infection, and the symptoms don’t show up in the tree for a year or more after infection. Meanwhile, the disease can be spread by ACP. Research is under way to develop early HLB detection so that infected trees can be rapidly removed.  Early detection will also allow researchers to more rapidly assess treatment programs for controlling not only the spread of the disease, but also possible cures or rootstocks or scion varieties that might have some resistance to the disease.

HLB infected tree showing mottling in one part of the canopy

Over the last two weeks Mary Lu Arpaia (Extension Specialist, UC Riverside) and David Obenland (Plant Physiologist, USDA-ARS) had the opportunity to visit Israel and spend several days looking at avocados including a visit with Zvi Mendel and Stanley Freeman, the lead researchers in Israel on the Polyphagous Shot Hole Borer (PSHB) and its Fusarium fungal symbiont.  This is their report.

So far the beetle is still largely confined to the central coastal region of Israel and the northern Negev.  The beetle has also been found in the Upper Galilee at Kibbutz Hagoshrim in avocado and on ornamental trees in other locations in this region which is quite far from the primary infested area.  The infested avocado trees have been destroyed but the beetle population already spread outside of the site of the initial infestation.  Interestingly, this find was with a grower who packs their avocados in the coastal area.  It is assumed that the beetles arrived in the bins originating from the infested area.  This is a reminder to California growers and packers that to minimize the spread of pests clean bins are essential.  The spread of avocado thrips and persea mite in California is also assumed to have been by bins containing vegetative material.

The Israeli researchers have continued searching for materials that will either control the beetle or the fungus.  They have had reasonable success in the lab when they test materials under controlled conditions but application out in the field is not effective.  There are no chemical treatments on the horizon that growers can use.

We visited infested avocado orchards in the Hefer Valley and the region southwest of the Carmel mountains (south of Haifa).  We visited a Reed orchard which is believed to have been infested approximately 5 to 6 years.  Three years ago this grove showed heavy infestation in the entire grove.  What we saw on our visit was severe limb dieback, many broken branches scattered on the orchard floor, dropped mature fruit and smaller than normal fruit size for the fruit remaining on the trees.  Signs of the beetle boring as evidenced by sugar exudates were easy to find wherever we looked.  Dr. Mendel told us that the grower is giving up on this orchard and plans to bulldoze the orchard after harvest.  We went on to see several other groves; in all except a 2-year-old orchard it was easy to find limb dieback, fallen fruit and sugar exudate up and down branches.  We were told that they do not often find infestation of young groves but when they do, it is usually on the base of the trunk (either rootstock or scion).

We visited a plot with some growers along with Leo Winer (an extension officer) and Udi Gafni (head of the research and development unit of GRANOT) where insecticide applications to infested trees had been made last fall (2012).  Unfortunately, signs of continued beetle activity were relatively easy to find.  The growers told us about seeing fruit shriveling as the branch dies back.  Since substantial fruit drop occurs of both mature and developing there is also an overall drop in productivity as an infestation spreads throughout a grove.  Growers are extremely concerned and frustrated that there are no control measures for the beetle.  Similar to California, Israel has historically used minimal pesticide sprays.  The growers know that in areas already infested that spray applications are key to their continued orchard viability. 

Avocado growers in Israel are also seeing problems with Botryosphaeria fungal infections.   We visited the northern Negev Desert area where avocados are grown.  We went to a large Hass orchard the grower is attempting to control this problem using phosphite injections.  We were able to see upper limb dieback and staining on the upper branches.   

In the meantime, Drs. Mendel and Freeman are continuing to study beetle biology and the behavior of the Fusarium fungal component.  Dr. Mendel is developing a method to raise the beetle in the laboratory.  This will be a important breakthrough since it will allow for a better understanding of the beetle life cycle.  They know that the female beetle once it flies and seeks a place to burrow has about a 48 hour window to successfully establish itself in the host plant since this is the time period it can survive without feeding.  We also learned that the beetle carries the Fusarium spores in its mycangium (a specialized structure at the back of its jaw), rather than hyphae.  The larvae and pupa do not have mycangium, only the adults.  While the larvae pupate, the Fusarium in the galleries sporulates and the emerging adult as it feeds picks up the Fusarium spores.  On a side note, Drs. Mendel and Freeman do not necessarily agree with our use of the name PHSB.  They argue that the beetle is monophagous (eats only one kind of food, Fusarium) but uses several tree species as hosts.  We will have to see how the final name for the beetle is ultimately settled among the insect taxonomists.

Finally, Dr. Mendel is very worried about the spread of the beetle to native tree species in Israel, especially oaks which are found in many areas throughout the country.  The box elder (which also occurs as a landscape tree in California) has been decimated by the PHSB.  This is a warning flag for all California residents to take action to safeguard our native oaks, other native species and landscape trees.

These visits reinforced the extreme importance that the California industry must be diligent looking for tree infestation.  The industry must work with the landscape industry and forestry service in southern California to understand how fast the infestation is spreading and what hosts are most susceptible.  The industry needs to continue funding both applied (surveys, control measures and understanding of the beetle and fungal biology) as well as more basic work such as the origin of the beetle.  This latter piece of information may lead to better ideas concerning control measures.  Finally California researchers and industry leaders would be well advised to collaborate closely with our Israeli colleagues.  They are facing this crisis head on since the PSHB has already spread into commercial groves.  From their advanced experience, the California industry can learn much.

A conference on the PSHB and its Fusarium fungal symbiont was held in August 2012 in Riverside.  The talks from this meeting are available for review on the website www.avocadosource.com,

Invasive Ambrosia Beetle Conference - The Situation in California ...

www.avocadosource.com/Journals/...2012/IABC_20120814_TOC.ht..

.  Video of the conference presentations from the public meeting are available for viewing on YouTube.  The science portion of the meeting will be posted on YouTube in the forthcoming weeks.

Below

Figures 1,2.  ‘Reed’ avocado tree in the Hefer Valley, Israel.  Note previous limb breakage and branch dieback due to infestation by the PSHB and its fungal symbiont, Fusarium. (Pictures taken 3/9/2013 by M.L. Arpaia).

Figure 3. Infested ‘Reed’ branch in later stages of decline.  The white larvae are termites.  Once the branch begins to dieback, termite infestation usually follows.  Note the staining of the wood and evidence of the PHSB galleries.  (Picture taken 3/9/2013 by M.L. Arpaia)

Figure 4.  External symptoms of infestation including sugar exudate and bark darkening reveals wood staining due to Fusarium infection of the woody tissue.  (Picture taken 3/9/2013 by M. L. Arpaia)

Figure 5.  Small branch infestation and dieback in ‘Hass’.  Picture taken at Kibbutz Ma’agan Michael along the central coast of Israel.  (Picture taken 3/9/2013 by M. L. Arpaia)

Figure 6. ‘Hass’ avocado tree in the northern Negev affected with Botryosphaeria.  Note the dieback and the fact that previously infested wood has been removed.  (Picture taken 3/9/2013 by M. L. Arpaia)