Botryosphaeria Gummosis and Dieback
DeAnna Vega, M.S. Candidate, CSU-Pomona
Botryosphaeria dieback or Botryosphaeria gummosis (formerly known as Dothiorella gummosis on citrus), is found worldwide on many horticultural and agricultural crops. Multiple species of the fungus can often be found on a tree at any given time. Bot canker has a latent stage where it can exist within a tree for years without showing any symptoms. Most species are opportunistic, relying on plants to become stressed before it can infect the tree. The fungus can infect the scion, rootstock, and surface roots of a tree. Disease symptoms include; necrosis of the blossoms, shoots, and fruit. Botryosphaeria can lead to wilt, leaf lesions/spots, and fruit rot. Cankers along the stem, twigs, and branches are sunken necrotic (dead) areas with gum exuding from the bark. Transverse section (cross section) exposes wedged or irregular stained sections of infection ranging in color from gray to brown. Infected cambial wood can appear brown to yellowish in color. Extensive dieback, even mortality in extreme cases particularly with younger trees, can occur. Some unique characteristics of Botryosphaeria are black pustules (pycnidia – a type of fungal fruiting body) grouped together on the surfaces of or heavily infested bark.
Figure 1. Gummosis on blighted twig – red arrows pointing to gummosis exudates (A).
Canker with older exude (B). Blighted foliage and fruit (C). Botryosphaeria
sp. on fruit (D). Blighted twig (E). Photos courtesy of Ben Faber.
The pathogen favors warm weather from 20°C - 30°C (68°F - 86°F) and rainfall. When, temperatures rise above 10°C (50°F), spores are released to spread the disease in spring. In California, new year's infection typically starts in early April and the middle of spring (late May through June) from previously infected buds, where the disease had not fully developed. Infected buds produce shoots that are infected with the disease. The infected areas can become blighted within three to five days. Symptomatic leaves turn chlorotic (taking on a yellowish and sometimes mottled appearance), wilt, and becomes blighted along with the twig. Flagging (blighted and dead branches) occurs throughout the canopy where the disease has infected.
Figure 2. Flagging in canopy from multiple blighted twigs. Photo courtesy of Ben Faber.
Control and management of the disease depends on cultural practices, chemical control, and integrated disease control management (Michailides & Morgan, 2004). Cultural disease control practices include; pruning, and timing of pruning, avoid pruning or mechanical damage during the rainy season. Prune cankered limbs two to five inches below diseased wood. Remove pruned away dead wood from the field prior to rainy/moist periods. Disinfect pruning tools with quarter strength household bleach – can be corrosive, 100% Lysol, or surface sterilize by flame. Use drip irrigation when possible and avoid wetting the trunk and canopy of trees. When necessary, irrigate with lower pressure to avoid misting and fogging that adds to humidity. Remove weeds to reduce the humidity and habitats for other insect pests. Insects, birds, and irrigation water can aid in disseminating the disease.
Be vigilant in scouting and remove the first sources of inoculation in summertime and repeat until disease is difficult to find. Remove and burn brush from the orchard as the disease can remain viable for as long as three to six years (Michailides & Teviotdale, 2014; Michailides & Morgan, 2004) in woody debris. When possible, protect tress from stressors (water, heat, mechanical injury, salinity, frost, nutrition deficiencies, or sunburn). Soil borne pathogens can predispose plants to Botryosphaeria gummosis by disrupting water and nutrient flow. Insect pests (such as scale) can also increase disease incidents as much as 50% or higher.
Integrated pest management is crucial to mitigating this disease. Buy clean nursery stock. Keep active monitoring regimes. Surface disinfect tools and equipment between cuts. Control other diseases (such as soilborn pathogens) and insect pests. Whitewash when necessary. Most importantly, follow proper irrigation and fertilizer regimens to ensure plant health.
and there's more:
Adesemoye, A. O., Eskalen, A., Faber, B., & O'Connell, N. (2011). Multiple Botryosphaeria species causing “Dothiorella” gummosis in citrus. Citrograph, 2(2), 32-34.
Faber, B. Citrus ID and Control. University of California Agriculture and Natural Resources Statewide Integrated Pest Management Program.
Michailides, T.J., Morgan, D. P., (2004). Panicle and Shoot Blight of Pistachio: A Major Threat to the California Pistachio Industry. American Phytopathological Society. http://www.apsnet.org/publications/apsnetfeatures/Pages/Pistachio.aspx
Michailides, T.J., Teviotdale, B.L., (2014). Pistachio: Botryosphaeria Panicle and Shoot Blight. University of California Agriculture and Natural Resources Statewide Integrated Pest Management Program. http://ipm.ucanr.edu/PMG/r605100311.html
On October 5, 2017, the California Avocado Commission (CAC) was alerted to a possible infestation of avocado lace bug (ALB; Pseudacysta perseae) in commercial avocado groves in Oceanside and De Luz, San Diego County. UCR's Mark Hoddle confirmed the infestation. During his survey, Hoddle found all life stages of ALB in groves in both locations, indicating a well-established, reproducing population.
Avocado Lace Bugs are an introduced (who did it?) pest that was found in San Diego in 2004. Previously in the US, it had only been found in Florida. Their impact of avocados can be minor, or with explosive growth can cause significant defoliation. ALB has been known to be established in southern San Diego County for about 15 years, but only on backyard avocado trees. It appears that this is the first infestation of this pest in commercial groves.
Feeding damage occurs near mid-vein on lower leaf surface and appears as a yellow halo on the upper leaf surface.
Lace bugs restrict their feeding to the undersides of leaves, inserting their needle-like mouthparts into leaf tissue cells to extract cell contents. Feeding initially causes small white or yellow spots on the surface of the leaves as individual cells dry out. It is suspected that feeding damage can provide entrance for pathogenic fungi, in particular Colletotrichum spp., which are leaf anthracnose fungi. As lace bug colonies grow, brown necrotic (dead) areas develop where there has been heavy feeding damage. These necrotic areas look like tip-burn caused by salt damage, but in this case, the necrotic areas are islands of dead tissue in the interior of the leaf surrounded by living tissue. Heavy feeding can cause striking leaf discoloration and early leaf drop.Other signs of lace bugs are dark, varnish-like excrement and shed white nymphal skins on the undersides of leaves. Avocado lace bug nymphs and adults do not feed on fruit
Avocado lace bugs have only been reported feeding on avocado, red bay, and camphor, all members of the Lauraceae family. Experimental evidence from Florida indicates that avocado varieties vary in their susceptibility to feeding damage. West Indian x Guatemalan avocado hybrids appear to be particularly resistant to attack in Florida. Observations in the Dominican Republic indicate that Hass avocados (a Mexican-Guatemalan hybrid) can be severely damaged by lace bug outbreaks with occasional severe infestations causing defoliation and reduced yields.
It's important that we all keep our eyes open for this pest. It can easily be confused with salt burn or leaf blight damage, though. Looking for the insect which can be present all year round, the excrement pellets and the unusual yellow halo feeding symptom on the upper leaf surface are all good indicators that the insect is there. Leaf Blight fungus on the other hand has dead spots that appear on both upper and lower leaf surfaces and have wave effect as the fungus spread beyond the point of origin.
For complete details on identification, life cycle and management, you are encouraged to visit the UC IPM ALB webpage (http://ipm.ucanr.edu/PMG/PESTNOTES/pn74134.html).
Look for adults and excrement pellets
Leaf Blight caused by Botryosphaeria sp.
A recent grower survey in Santa Barbara County asked a whole bunch of questions. One of which was had they had an evaluation of irrigation distribution uniformity. This is a free service that can significantly improve on-farm water use and most importantly improve plant health. Avocados that don't get the right amount of water at the right time are extremely susceptible to root rot. Proper irrigation is the first line of defense against root rot, good farming that results in good economic returns to the grower.
So, with a free DU available to growers, how many do you think took advantage of the service? Barely 50%!!!!!!!! This just does not make sense. In a land of little water and frequent examples of what can happen with no water ………………..and high priced water, what is going on?
The local Resource Conservation District has done many system evaluations, and most results find that improvements can be made in distribution uniformity. This is true in relatively new irrigation installations. It does not take long for problems to occur in even well designed and installed systems.
During the summer of 2007, the Casitas Municipal Water District (CMWD) contracted with the Irrigation Training and Research Center (ITRC) of California Polytechnic State University, San Luis Obispo, to conduct field evaluations of drip/micro systems. A team of two students conducted 35 field evaluations.
Distribution Uniformity (DU) – DU is a measure of the uniformity of water application to trees throughout an orchard, with DU = 1.0 being perfect. The measured orchard DUs in the Santa Barbara/Ventura area had an average DU of 0.66, while the California state average for drip/micro is 0.85.
In general, there were substantial opportunities to improve the distribution uniformity (DU) of the water to trees throughout an orchard. An improved DU will minimize over-irrigation in some areas, and reduce under-irrigation in others. Key recommendations that were provided included:
Install a pressure regulator at the head of every hose
With a regular microsprinkler, doubling the pressure causes about 40 percent more water to come out of the nozzle. Pressure regulators are added to have similar pressures throughout the orchard and thus reduce the risk of over-irrigating portions of the field. On many farms, the difference between the highest pressures was double or even triple the lowest pressures (40-70% more water). By adding the correct high-quality, pre-set pressure regulators with the correct flow rate rating, the farmer can get similar pressures to every nozzle and prevent over-irrigation.
For a pressure regulator (PR) to work, more pressure must enter the PR than what the PR is rated for. For example, to use a 25 psi PR, you need at least 27 psi into the PR. All a PR does is reduce pressure; it cannot add pressure.
Another problem on hillsides is that some pipes have as much as 100 psi before the PR. A PR can effectively reduce the pressure down to 50%. What is recommended in these fields is to reduce the pressure in the pipe by adding an in-line valve halfway down the hill and throttling it down to a reasonable pressure.
Completely replace all microsprinkers with pressure compensating microsprinklers
Pressure compensating microsprinklers have an internal flexible diaphragm that reduces a pathway as the pressure increases. These allow similar amounts of water to get the trees even if the hoses do not have the same pressures. Whenever the pressure is doubled, 10 percent more water will come out of these emitters, compared to 40 percent more water with a regular microsprinkler. Having pressure compensating emitters can drastically improve the DU in virtually every avocado orchard because most irrigation systems were not properly designed for microsprinkler systems, or because the farmer has altered the original design by adding different-sized nozzles.
Reduce plugging problems
Major plugging problems are found in all orchards that did not have good filtration, even those that get district water. There were also some “within-system” causes of plugging. Almost all plugging is from simple dirt or rust, as opposed to bacteria or algae. Recommendations are as follows:
- Always have a filter at the head of the system. The required mesh size depends on the microsprinkler flow rate, but 120 mesh is a starting point.
- Remove hose screen washers that are found at the head of hoses, and replace them with regular washers (after installing a filter at the head of the system). The hose screen washers often plug up and cause the hoses to have unequal inlet pressures.
- Be sure to thoroughly flush hoses after any hose breaks.
- Double check the type of fertilizer that is being injected, especially any “organic fertilizers”. Some of these can plug emitters. In any case, inject the fertilizers upstream of the filters. If the filter plugs up, it is better to have discovered the problem early.
- Clean the filters frequently. Install pressure gauges upstream and downstream. When the pressure differential (as compared to a clean screen) increases by 3-5 psi, it's time to clean the screen.
In some orchards, there is a big plugging problem caused by insects crawling into emitters after the water is shut off. Many of the new microsprinkler designs utilize a self-closing mechanism to prevent insects from coming into the nozzle.
We have gotten a reprieve with the rains and refilled reservoirs, but it is ever more important to make sure our irrigation systems are doing what they are supposed to be doing. Call your local Resource Conservation District and get information about a system evaluation. Contact numbers can be found at: http://www.carcd.org/rcd_directory0.aspx
Get ready for more rotting avocado fruit if you have leaf blight showing up in your tree canopy. The fungal spores (one of the Botryosphaerias we once lumped as Dothiorella) that create the infection spread in an irregular pattern over the leaf and down the stem (then called “stem blight”). This is often confused with salt or tip burn. The two conditions are caused by the same problem, water and or salt stress. However, in the case of leaf blight, this is a pathogen that can pass to neighboring fruit and begin the process of rot. This starts happening when the fruit starts ripening and softening, so it's often not seen in the orchard, but the packhouse or in the market.
Control is basically gaining control over the soil moisture and salinity in the root zone and when the leaf blight starts showing up in the canopy, cutting as much out back to green tissue as is economically possible.
Leaf blights from this group of fungi have also been reported as infecting other fruits, such as citrus, apple, peach and grape among others. The solution is the same - water right and cut the stuff out when and if it shows up.
Rot spreading to flesh
A 'Meyer' lemon should be quite happy along the coast, unless it gets planted in new soil that has low copper because of high soil pH or high organic matter. And then you wonder what is wrong.
Mild copper deficiency is usually associated with large, dark green leaves on long soft angular shoots. Young shoots may develop into branches which appear curved or “S-shaped," referred to as “ammoniation” usually resulting from excessive nitrogen fertilization. It's actually thought to be too much nitrogen relative to copper in the plant and can be corrected by foliar sprays. Twigs can develop blister-like pockets of clear gum at nodes. As twigs mature, reddish brown eruptions may occur in the outer portion of the wood. It can be quite shocking. Severely affected twigs commonly die back from the tip with new growth appearing as multiple buds or “witches broom”. Necrotic-corky areas on the fruit surface may sometimes occur in extreme situations. In some cases fruit cracking occurs with exudates.
Copper deficiency is more likely to occur in new plantings on previously uncropped soils, which are usually deficient or totally lacking in copper. In California, it has been referred to as “corral” disease or “midden” disease because it is associated with high organic matter that ties up the copper, or old Native American sites were debris had been piled. It is often localized in certain areas. Once I saw it on nursery trees that had had inadequate copper in the nutrient solution. I've only seen it on citrus, and not any other subtropical like avocado, but that doesn't mean it can't happen.
I've also seen gummosis similar to this occurring with drought and water management. It more commonly occurs as a twig die back at the tips. And certainly Phytophthora gummosis will show gumming. It's that little gumming pocket under the bark is usually the way to distinguish copper deficiency from these two others.
Pictures: pocket gumming (U. of Florida), oozing (Yara), gumming, and more gumming not a worm