Up-front grove management techniques including pathogen-free propagation material and fallowing ground or rotating to annual crops prior to replanting are effective tools to help prevent citrus diseases, according to Mike Matheron, Extension plant pathologist with the University of Arizona.

There are about 150 citrus diseases worldwide. Arizona growers combat 16 diseases.

“Extreme heat impacts a pathogen's ability to survive and infect plants,” said Matheron, who is based at the Yuma Agricultural Center, Yuma, Ariz. “Pathogens that cause exotic citrus diseases don't exist in the low desert. If a pathogen was introduced, environmental factors could very well prohibit disease development.”

Matheron discussed citrus diseases with growers at a workshop in Phoenix, Ariz. His comments were based on low desert citrus production areas including Yuma, Ariz., central Arizona, and California's Imperial and Coachella valleys including the Blythe area.

Matheron says fungi or fungi-related organisms cause about half of Arizona's citrus disease problems including the Phytophthora-caused root rot, gummosis, and brown rot.

Others problems include Hendersonula branch wilt (sooty canker), lemon tree wood rot, dry root rot, blue and green fruit molds, Alternaria fruit rot, and Rio Grande gummosis.

Diseases caused by non-fungi pathogens include slow decline, stubborn disease, citrus tristeza, and psorosis.

Matheron says the development of citrus disease requires three main ingredients: a host plant, virulent pathogen, and a favorable environment for host and pathogen interaction to initiate the disease.

“Sometimes a disease requires a fourth component; a vector to introduce the pathogen into the host plant,” Matheron said. An example is the Asian citrus psyllid insect, the primary vector for citrus greening (Huanglongbing) disease currently found in Florida and Louisiana.

The psyllid has been confirmed in California's San Diego and Imperial counties, but not in Arizona. Neither state has a confirmed case of citrus greening.

Phytophthora and the citrus nematode cause the most problems for Arizona citrus growers. While Phytophthora was considered a fungus for years, genetic analysis has confirmed a closer similarity to algae.

Phytophthora root rot, gummosis, and brown rot are very common in Arizona. The Phytophthora pathogen is the causal agent. Matheron says virtually every citrus operation has the Phytophthora nicotianae (P. parasitica) and/or Phytophthora citrophthora parasitica organisms in the soil.

“The insidious part of Phytophthora is that the organism's life cycle exists below the ground,” Matheron said. “The only clues to the organism's presence are finding damaged roots or gummosis on the tree trunk.”

To manage citrus diseases, Matheron encourages growers to “know their enemy” by understanding the Phytophthora life cycle. Small structures called zoospores initiate the infection. Water added to the soil through irrigation or rain triggers the pathogen's ability to reproduce.

“With flood irrigation these little guys can go a long distance,” Matheron said. “Even without flooding the organisms move through pores in the soil.”

Matheron says the best management techniques for Phytophthora include a pre-plant soil treatment with the soil fumigant Vapam, resistant rootstocks, not overwatering, enhanced soil drainage, and fungicides including Ridomil and Aliette.

When groves are to be replanted, Matheron recommends fallowing the grove through the summer to allow the summer heat to sterilize the soil and replant citrus in the fall.

Slow decline disease, caused by the citrus nematode, Tylenchulus semipenetrans, causes swollen roots. The nematode burrows halfway into the roots where eggs are laid which grow into larvae. Only the females burrow in the roots.

The nematode bandit robs the tree of vital nutrients. Higher nematode numbers reduce the nutrition from the roots to the tree causing slow tree decline including yellowed leaves. Eventually fruit yield and quality are reduced.

Best management techniques include managing nematodes prior to tree planting, purchasing nematode-free nursery stock, soil fumigation before replanting, and fallowing the ground or rotating to annual crops before replanting with citrus.

Dry root rot disease is caused by the fungus Fusarium solani which results in trees drying up in the mid to late summer. While the disease seldom kills a healthy, developing tree, death can occur when the tree is under other stresses, Matheron says. The disease is found primarily in the Yuma area.

Successful management tips include eliminating stress factors that predispose trees to disease, proper fertilization, adequate soil drainage/aeration, healthy roots, and proper watering.

Lemon tree wood rot is caused by the fungi Antrodia sinuosa, Coniophora eremophila, and Nodulisporium sp. The fungi are most active from May through October.

Lemon tree wood rot originates in the branches. The first sign of rotting is the death of a small branch followed by major limbs breaking off.

Lemon tree wood rot is most common in the Yuma area and has been found in the Phoenix area and southeastern California. The high incidence in Yuma could be partially due to the abundance of lemon trees compared to other areas. Antrodia is the most prevalent pathogen in Yuma area lemon groves.

Successful management: remove infected branches from the tree early and destroy the infected wood to reduce the potential of new infections.

Citrus stubborn disease is caused by the phytoplasma Spiroplasma citri which results in stunted canopy and small, lopsided fruit or no fruit at all. The leaves are small and grow upright close to the stems. The pathogen is spread by leafhopper feeding, primarily the beet leafhopper, plus by grafting and budding.

Citrus stubborn primarily affects sweet orange, grapefruit, and tangelo trees.

Successful management: preventive measures mainly apply to nursery practices including stubborn-free mother trees for budwood.

“If you're a nurseryman, make sure trees in the nursery are not infected,” Matheron said. “Once in the grove, there's really not much to be done to prevent the disease from spreading.”

The Tristeza virus is to blame for the Tristeza disease complex which results in tree decline. Tristeza is spread through budding and grafting or by aphids feeding on citrus. Tristeza diseases, including quick decline, seedling yellows, and stem pitting, are caused by different isolates of the tristeza virus.

Successful management techniques include tolerant rootstocks and tristeza-free propagation material. Aphid control will not significantly reduce virus spread in commercial groves since the aphid would usually acquire and transmit the pathogen before being killed by an insecticide.

Psorosis, caused by the Citrus psorosis ophiovirus, kills the main scaffold branches in mostly older orange and grapefruit trees. The most distinguishing symptom is the scaling and flaking of the bark on the scion.

“A tree with psorosis usually will be less productive than healthy trees; tree replacement is the best option,” Matheron said.

The use of disease-free budwood is the best management technique.

Citrus research

Matheron is in his third year of research on the possible effect of rootstocks and lemon varieties on the development of lemon wood rot by Antrodia sinuosa.

Research results in 2006 and 2007 revealed that wood decay occurring after branch inoculation in five lemon varieties (Corona Foothills, Eureka, Frost, Limoneira 8A, and Prior) established on Citrus macrophylla rootstock was significantly higher compared to lemon varieties established on rough lemon and Citrus volkameriana rootstocks, Matheron says.

No consistent differences were found in the different lemon varieties on the same rootstock. The third-year trial will be wrapped up in April 2009.

The research results suggest that wood rot will develop on lemon established on all three of the tested rootstocks, Matheron says. However, the rate of wood decay for trees on C. macrophylla rootstock will be higher.