- Author: Ben Faber
Along the coast, it is very common to see windbreaks protecting the citrus and avocado groves. Invariably the first two rows next to the eucalyptus trees are shorter and less thrifty than the citrus further away from the windbreak. This is due to competition primarily for water, but somewhat due to light, as well. Often by putting emitters on the windbreak, the completion stops. Growers will also root prune between the windbreak and the first row of citrus. Those roots inevitably grow back and pruning must be done again. This also occurs in areas where there are oak trees or other natives that are planted in or around the orchard. Growers will frequently plant right up to the canopy or even under the canopy of the native tree(s), with a similar result seen with windbreaks.
It is important to remember the architecture of roots. Not all trees are exactly alike, but a general rule of thumb is that the active roots go out one and half times the height of the tree. So a 40 foot tree will have competitive roots out 60 feet away from the trunk. That’s why it is best to keep a distance away from a competing tree, because avocados and citrus are just not as competitive as an oak or eucalyptus.
In low rainfall years, this competition is even more intense. Significant defoliation of the crop plant can be seen. The grower then thinks that it is some disease and ponders what to spray, when they should actually be spraying more water.
- Author: Ben Faber
A microirrigaiton workshop on use and maintainence of systems in Spanish is being held on three successive dates in San Luis Obispo, Ventura and Fallbrook. These are the same workshop held on July 23, 24 and 25 in the respective areas. The workshops are intended for Spanish speaking workers who maintain and use microirrigation systems. The classes are free and no registration is required. These and future workshops are funded by USDA. For more information contact Lesa Scarborough at:
administration@californiaavocadosociety.org
951-225-9102 Office
951-225-9198 FAX
Click on the flyer below for more details
Irrigation flyer
- Author: Akif Eskalen and Virginia McDonald
Branch and trunk canker on avocado was formerly attributed to Dothiorella gregaria, hence the name Dothiorella canker. So far Botryopshaeria dothidea (anamorph: Fusicoccum aesculi) is the only known species causing Dothiorella canker on avocado in California. Symptoms observed on avocado with Dothiorella canker include shoot blight and dieback, leaf scorch, fruit rot, and cankers on branches and bark.
However, recent studies based on DNA analyses suggest greater species diversity of this pathogen group than based on morphological characteristics alone. Thus far, multiple species of Botryosphaeriaceae have been found to cause the typical Dothiorella canker (Fig3.) and stem-end rot (Fig 5) on avocado in California. Percent recovery of Botryosphaeria spp. based on morphological characters ranged from 40-100% in Riverside county, 42-53% in Ventura county, 33% in Santa Barbara county, 60% in San Diego county and 32-60% in San Luis Obispo county.
According to preliminary results from a continuing survey throughout avocado growing areas of California, multiple species of Botryosphaeria (Neofusicoccum australe, B. dothidea, N. luteum, and N. parvum) were found.
Pycnidia (overwintering structure) of Botryosphaeriaceae species were also observed on old diseased avocado tree branches. Sequenced rDNA fragments (ITS1, 5.8S rDNA, ITS2, amplified with ITS4 and ITS5 primers) were compared with sequences deposited in GenBank.
Pathogenicity tests were conducted in the greenhouse on 1-year-old avocado seedlings, Hass cv., with one randomly chosen isolate from each of the Botryosphaeriaceae species noted above. Four replicate seedlings were stem-wound inoculated with a mycelial plug and covered with Parafilm. Sterile PDA plugs were applied to four seedlings as a control. Over a period of 6 months, seedlings were assessed for disease symptoms that included browning of leaf edges and shoot dieback. Mean vascular lesion lengths on stems were 64, 66, 64, and 18 mm for B. dothidea, N. parvum, N. luteum, and N. australe, respectively. Each fungal isolate was consistently reisolated from inoculated seedlings, thus completing the pathogenicity test. To our knowledge, this is the first report of N. australe, N. luteum, and N. parvum recovered from branch cankers on avocado in California.
These results are significant because Botryosphaeriaceae canker pathogens are known to enter the host plant through fresh wounds (pruning, frost, and mechanical). With high-density planting becoming more common, which requires intensive pruning, the transmission rate of these pathogens could increase in California avocado groves. The Eskalen laboratory is currently investigating control measures for dothiorella canker and stem-end rot pathogens.
Branch dieback and trunk canker caused by the fungus
- Author: Akif Eskalen, Richard Stouthamer, S.C. Lynch, M. Twizeyimana, A. Gonzalez and T. Thibault
The polyphagus shot hole borer (PSHB) is an invasive beerle that forms a symbiosis with a new, as-yet-undescribed Fusarium sp., together causing Fusarium dieback on avocado and hother host plants in California and Israel. In California, PSHB was frist reported on black locust in 2003, but there were no reords of fungal damage until 2012, when Fusarium was recovered from the tissues of several bakyard avocado trees infested with PSHB in Los Angeles County. A study was conducted recently to determine the plant host range of the beerle-fungus complex in two heavily infested botanical gardens in Los Angeles County. Of the 335 tree species observed, 207 representing 58 plant families, showed signs and symptoms consisten with attack by PSHB. The fungs was revcovered from 54% of the plant species attacked by PSHB, indcated by the presence of the fungus at the site of the enty hole. Trees attacked by PSHB included 11 species of California natives, 13 agriculturally important species and many common street trees. Survey results also revealed 19 tree species that function as reproductive hosts for PSHB. Additionally, approximately a quarter of all tree indiviuals planted along the streets of southern California belong to a species classified as a reproductive host. The results suggest that the pest-disease compex could establish in a variety of plant communities locally and worldwide.
- Author: Ben Faber
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.