Avocado tree Pest and Diseases – Avocado Weed Management

Avocado tree

George Marakas

Fruit (Avocado) producer and trader

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In general, avocado is not threatened by many pests and diseases, and usually, no management measures are applied, making the organic cultivation of the crop possible. Periodically, in my region (Crete, Greece), the increase of thrips population may cause problems and damage the fruits.

Avocado Pests and their Management methods

Avocado Thrips (Scirtothrips perseae)

Thrips are a key pest in most orchards worldwide that scars the avocado fruit. The adults are very small (0.7 mm or 0.03 in. long), with an orange-yellow coloring (brown bands on their body and three red dots on the top of the head), and fringed-tipped wings. Its population increases in (late) spring, moving from the foliage to the young fruits. Depending on the temperature (ideally 18-24oC or 65-75 oF), the insect may have more than six generations per year.

While the internal fruit quality is not affected, the insect can severely scar the skin. Almost all damage occurs when fruits are 5–15 mm (0.2 to 0.6 in) long. This “alligator skin” is not commercially acceptable, leading to the avocado’s downgrading (B quality) and extended economic loss for the farmer. Based on experimental results, 3-5 thrips per leaf can cause up to 38% fruit damage. If the population is under control by insects’ natural enemies, or/and slight damage occurs very early in the fruit setting, there is a possibility that the scars will not be so apparent as avocado enlarges (especially Hass avocados).

If the problem persists and pest control is needed, the grower can apply selective insecticides, always after discussing with a local licensed agronomist. Application of products with abamectin can control thrips and persea mites at the same time. Some other practical options are Veratran D + Sugar/Molasses, Success 2 SC + Narrow Range 415 Spray Oil, and Agri-Mek 0.15 EC + Narrow Range 415 Spray Oil (1). However, it is better to protect the natural enemies of the avocado thrip, like predatory thrips (Franklinothrips orizabensis or F. vespiformis) that can generally keep the insect under control. Finally, modifying fertilization and pruning reduces the growth of tender new foliage, which is attractive to thrips (2).

Other common pests of primary concern for avocado are the: Amorbia (Western Avocado Leafroller), Avocado Brown Mite, Greenhouse Thrips, Omnivorous Looper, Persea Mite, Polyphagous Shothole Borer, and Kuroshio Shothole Borer, Sixspotted Mite. Aphids, White Flies, Cutworms, and leaf miners are more likely to become a problem in the avocado plant nursery. Since plants are susceptible to such infestations at the seedling stage, the staff responsible must take all prevention and control measures. Additionally, young avocado plants in the field are in danger from Branch and Twig Borer, European Earwig, False Chinch Bug, Fuller Rose Beetle, Grasshoppers, and June Beetles (3).

In some countries, snails and rodents (like gophers and ground squirrels) may also become a persistent problem for avocado growers. More specifically, gophers can damage sprinklers, divert the irrigation water due to tunnel digging, and damage the young avocado plants. Control by traps is sometimes needed, especially in the first three years of the avocado orchard establishment (4).

The major avocado diseases and how to control them

While no significant problems have been reported in Greece, Phytophthora Root Rot is considered the most common and catastrophic avocado disease, putting global production at risk. At the same time, Anthracnose can cause quality decrease and extensive fruit losses, especially post-harvest.

Phytophthora Root Rot (Phytophthora cinnamomi)

It is the most serious and globally spread disease of avocado, leading to plant death. In some areas of Mexico, the pathogen has infected up to 90% of avocado trees. In Andalusia (Spain), this number is close to 40% (5). The Phytophthora cinnamomi is a soil-born fungus that can infect the plants’ roots that eventually rot. This significantly decreases the capability of the plant to absorb nutrients and water. The first obvious symptom is the development of cachectic and smaller leaves. The plants start losing their leaves, wilt, and eventually die.

The most important preventive measures are the use of healthy, certified plant material and the selection of a variety grafted in a resistant to the pathogen rootstock. The Dusa rootstock is considered one of the most tolerant ones to avocado root rot disease. Excellent level of resistance to Phytophthora cinnamomi exhibits the Duke 7, Toro Canyon, Latas, Martin Grande, Thomas, Uzi (PP15), and Zentmyer (PP4) rootstocks. They are exclusively produced with clonal propagation since the resistance to the pathogen is not transmitted to the new generation by the seed (6).

Prevention is the best and most effective solution since the actual control of the pathogen is extremely difficult and demands a combination of measures (integrated management). Poorly drained and waterlogged conditions accelerate the spread of the diseases and the severity of the symptoms. As a result, it is vital to select well-grained fields to plant an avocado orchard, take all measures needed to ensure that water lodging will be avoided, and prefer controlled drip irrigation of the orchard. Furthermore, good nutritional practices, including sufficient calcium, can be helpful.

If the farmer and agronomist decide that the disease’s chemical control is necessary, the best option is often Phosphonate fungicides (7). The application should be performed on an annual or biannual basis. On the other hand, Metalaxyl (Ridomil) mixed with soil before planting or applied as a soil drench controls root rot at least for four months after treatment (8). Such application can allow replanting of avocado trees (8 weeks after) in a spot with known disease history.

Anthracnose (Colletotrichum gloeosporioides)

This fungal disease can lead to extensive quality loss, especially in warm, humid climates. Some common symptoms are the dry, dark-brown spots that are formed on the fruit skin. In many cases, the symptoms are “masked” by the thick dark skin of some varieties like Hass and are detected only by the consumer that opens the fruit. Spots on the leaves or dead leaves can also be an indication. In severe infections, the farmer can observe abnormal fruit development or fruit drop (9). The disease can spread with the seed, crop residues, and water droplets. Fuerte, Rincon, and Wurtz scion cultivars are very susceptible to the disease (10). In areas with extensive problems, farmers should prefer varieties with thick skin like Sharwil (Guatemalan with some Mexican genes), which has a good tolerance (11).

Management measures include regular sprays (every 2-4 weeks depending on weather conditions) from fruit set initiation until harvest using a registered protectant fungicide such as copper oxychloride. Moreover, the farmer can cut and destroy dead and infected plant parts, prune and harvest only when the conditions are dry, and improve the aeration of the tree canopy. Finally, to avoid disease dispersal and slow down the fruit deterioration, it is essential to cool them down at 5.5 oC (42 oF) after harvest and store them dry (10).

Other common diseases affecting avocado are the: Armillaria Root Rot (Oak Root Fungus), Avocado Black Streak, Bacterial Canker, Branch Canker and Dieback (Formerly Dothiorella Canker), Fruit and Stem-End Rots, Fusarium Dieback, Phytophthora Fruit Rot, Phytophthora Trunk Canker and Crown Rot (Formerly Citricola Canker), Rosellinia Root Rot, Sooty Mold, Verticillium Wilt and Sunblotch (3).

Finally, removing and destroying diseased plant materials can help prevent the spread of many diseases. Most farmers use a fungicide, copper, or azoxystrobin approved for the crop to control many of the diseases mentioned above. The grower should consult an experienced licensed agronomist and follow the manufacturer’s directions for every product.

Avocado Weed Management

Weeds compete with the trees for nutrients and water, while they can host important pests and diseases of the crop. In a healthy, mature, and well-established avocado orchard, weeds do not usually cause significant problems. However, if the plant distances are large and there is a lot of free and sunny space for the weeds to grow, or/and there is water scarcity, the farmer may need to apply weed management techniques. Successfully controlling weeds is vital during the planting of the orchard and at least for the first 3-4 years. The three methods of weed control that are available for avocado are mowing, tillage, and the use of herbicides.

To reduce the nutrient and water losses in my fields, I choose to apply weed control one or two times per year, even in my orchards with older trees. Many fellow farmers use a tractor with a milling machine to destroy weeds. Unfortunately, in most of my fields, the machines cannot move around quickly, and weed control is mainly executed by hand. Especially in new plantations, some growers prefer to apply herbicides. The herbicide can be sprayed three times annually (during February, May, and August for the north hemisphere) (12). In this case, farmers need to be very careful and avoid excessive and careless use to decrease the risk of herbicide-resistant weeds and damage to the avocado trees (caused by using broadleaf target herbicides) (13).

Depending on the region and the field, the species and the populations of the weeds may vary significantly. Some of the most common weeds observed in avocado orchards are: Barnyardgrass, Bermudagrass, Crabgrasses, Puncturevine, Field Bindweed, Longspine Sandbur, Nutsedges, Dallisgrass, and Wild Cucumber. The last two species can be a significant problem for young avocado plants (14). Since many of these weeds reproduce with underground stolons and rhizomes, avoiding using a tractor with a milling machine is recommende because it can chop them into pieces and spread the problem in the entire field quickly. On the other hand, others produce a thousand seeds per plant that can spread and survive in the soil for decades, making their elimination from the field almost impossible. I would advise avocado farmers to scout their fields, recognize the weeds growing, learn about their physiology and then choose the most appropriate weed management strategy.

References

  1. http://www.avocadosource.com/papers/research_articles/hoddlemarkpdf
  2. https://wwwipm.ucanr.edu/agriculture/avocado/Avocadothrips/
  3. https://wwwipm.ucanr.edu/agriculture/avocado/
  4. https://coststudyfiles.ucdavis.edu/uploads/cs_public/b4/3d/b43d58d9-1e91-4a3e-80f9-a2edb14958b0/2020avocadohighdensitysandiegocounty.pdf
  5. https://www.elgo.gr/images/ioanna/periodiko/Teyxos_17/%CF%83%CE%B5%CE%BB._8-10.pdf
  6. https://wwwipm.ucanr.edu/agriculture/avocado/RootstockTolerancetoDisordersandPathogens/
  7. https://www.daf.qld.gov.au/businesspriorities/agriculture/plants/fruitvegetable/fruitvegetablecrops/avocado/pestsanddiseasesofavocadocrops
  8. https://www.fao.org/3/X6902E/x6902ehtm
  9. https://www.jica.go.jp/project/english/kenya/015/materials/c8h0vm0000f7o8cjatt/materialspdf
  10. https://wwwipm.ucanr.edu/agriculture/avocado/Anthracnose/
  11. https://link.springer.com/content/pdf/10.1007%2F978-3-319-06904-3.pdf
  12. https://coststudyfiles.ucdavis.edu/uploads/cs_public/b4/3d/b43d58d9-1e91-4a3e-80f9-a2edb14958b0/2020avocadohighdensitysandiegocounty.pdf
  13. http://www.avocadosource.com/CAS_Yearbooks/CAS_43_1959/CAS_1959_PG_75-78.pdf
  14. https://wwwipm.ucanr.edu/agriculture/avocado/SpecialWeedProblems/
  15. https://www.elgo.gr/images/ioanna/periodiko/Teyxos_22/AVOCADO.PDF

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Avocado tree Pest and Diseases – Avocado Weed Management

This post is also available in: Ελληνικά

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